1. A synchronous control device that makes a change in an angle of a master shaft or time elapsed from reference time change amount information and controls execution of an operation instruction of a slave shaft in synchronization with the change amount information, the synchronous control device comprising as a controller: a change amount information calculator that calculates the change amount information; an operation instruction execution controller that, when a start condition indicating an angle condition of the master shaft or a time condition from the reference time and a predetermined signal condition are defined for the operation instruction of the slave shaft and when the change amount information satisfies the start condition and the signal condition is true, determines a start of a corresponding operation instruction, and determines operation completion based on a completion condition determined in advance of the operation instruction; and an operation instruction executor that executes the operation instruction the start of which is determined by the operation instruction execution controller and stops the operation instruction when the operation completion is determined by the operation instruction execution controller.