1. A logic graph displaying apparatus for use in a molding machine that operates in accordance with on/off signals, the apparatus comprising: a memory; a control unit configured to acquire all on/off data generated by a molding sequence performed by the molding machine, the on/off data being associated with a plurality of signal identifiers, and to store, in the memory, logging data containing on/off state and timing data representing the time each signal identifier is switched on or off; and 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; wherein the control unit is further configured to: display an input window in which to input a signal identifier, on the logging screen, when the identifier display part displaying the signal identifier on the logging screen is switched on; display 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; read logging data associated with the signal identifier from the memory; and display the logic graph of the signal identifier in the logic-graph display part displayed on the logging screen.