A logging data about a signal identifier of the I/O signals which changed in on/off state are stored in a ring buffer. A logging screen includes identifier display parts, logic-graph display parts and first and second cursors. The logic-graph display parts display logic graphs associated with signal identifiers, respectively. When any one of the identifier display parts displayed on the logging screen is clicked, the logging screen is changed to an input window. If a desired input identifier is input in the input window, the logging data associated with the signal identifier is read from the ring buffer and the logic graph is displayed in the logic-graph display parts. The logic graph can be modified.