To provide a display unit which can easily investigate the cause of a trouble of a programmable controller.

The display unit which is connected to the programmable controller B controlling external equipment to display the operation state of the external equipment and also operates the external equipment, is provided with an investigating software storage memory 7 having stored trouble cause investigating software 71 for investigating the cause of trouble of the programmable controller B in such a case, a personal computer where cause investigating tool software for PLC has been installed is prepared, and the trouble cause is easily investigated without any connection.