1. An automatic welding device having an optimal welding condition operation means for setting information regarding a to-be-welded object that includes board thickness, outer dimensions, and joint shape of the to-be-welded object and for performing arithmetical operations on welding conditions that include welding current, welding voltage, and welding speed suitable for the information, comprising: temperature distribution operation means for performing arithmetical operations on a temperature distribution in a joint of the to-be-welded object from the information regarding the to-be-welded object and from the welding conditions; temperature distribution operation result recording means for recording at least the information regarding the to-be-welded object, the welding conditions, and a temperature distribution operation result calculated by the temperature distribution operation means; weld penetration display means for displaying at least a penetration width, a penetration depth, and a penetration shape based on the temperature distribution operation result; bead surface shape operation means for performing arithmetical operations on a molten-metal bead surface shape from at least the information regarding the to-be-welded object, the welding conditions, and the temperature distribution operation result that have been recorded in the temperature distribution operation result recording means; bead surface shape operation result recording means for recording a molten-metal bead surface shape operation result calculated by the bead surface shape operation means; bead surface shape operation result display means for displaying the surface shape operation result calculated by the bead surface shape operation means in the form of at least a cross-sectional view, a longitudinal sectional view, and a general view of a welded joint; bead shape observation position setting means capable of setting a bead shape observation position by a viewing point from an arbitrary position in displaying the bead surface shape; arbitrary position bead shape display means for displaying at least the general view of the welded joint from a position set by the bead shape observation position setting means; joint quality acceptance/rejection judging means for judging acceptance or rejection of a joint quality of the to-be-welded object from a temperature distribution in the joint of the to-be-welded object in the temperature distribution operation result recording means; weld starting means for automatically starting to weld the to-be-welded object under the welding conditions recorded in the temperature distribution operation result recording means when the joint quality acceptance/rejection judging means judges the joint quality to be acceptable; welding condition correction means for correcting the welding conditions of at least the welding current, the welding voltage, and the welding speed so as to satisfy acceptance criteria when the joint quality acceptance/rejection judging means judges the joint quality to be rejectable; operation result record situation judging means for judging the presence or absence of similar information regarding the to-be-welded object, a similar temperature distribution operation result record, and a similar bead surface shape operation result record before setting information regarding the to-be-welded object; and operation time omitting means for omitting time required for operation by the temperature distribution operation means and time required for operation by the bead surface shape operation means by allowing the weld penetration display means to display a temperature distribution operation result record in the joint of the to-be-welded object and by allowing the bead surface shape display means to display a bead surface shape operation result record when the operation result record situation judging means judges that there is an arithmetical operation record in the past.