PURPOSE: To decide and analyze the correlation among tasks and the executing time of each task by changing the contents stored in a memory into a name that can be understood by an operator decided previously and displaying the executing situation data on each task with no mutual overlapping based on the value of a system timer.

CONSTITUTION: The task executing situation data are stored in a memory 4 based on a multi-task executing situation data trace program 1, and the contents stored in the memory 4 are changed into a name that can be under stood by an operator based on a display form conversion/display program 2 with a display instruction given from a keyboard 8. Then the executing situation data on each task are displayed with no mutual overlapping based on the value of a system timer. Therefore the system timer value obtained at occurrence of an event is shown in the vertical direction and the contents of the event are shown in a names that can be understood by the operator. Then the tasks are shown in a row and in the name that can be understood by the operator, and the parameter data obtained at occurrence of each event are shown. Thus the faults, etc., are analyzed at a high speed.