1. A logic graph displaying apparatus for use in a molding machine that operates in accordance with on/off signals, the apparatus comprising: means for acquiring on/off data about a plurality of signal identifiers of the molding machine and for storing, in a memory, logging data containing on/off state and timing data representing the time each signal identifier is switched on or off; a logging screen having identifier display parts for displaying the signal identifiers and logic-graph display parts for displaying logic graphs of the signal identifiers; means for displaying an input window in which to input a signal identifier, when the identifier display part displaying the signal identifier on the logging screen is switched on; and means for displaying a signal identifier in the identifier display part on the logging screen when the signal identifier is input, in an input row provided in the input window, for reading the logging data associated with the signal identifier from the memory and for displaying the logic graph of the signal identifier in the logic-graph display part displayed on the logging screen.