PURPOSE: To enhance the efficiency of debugging by easily confirming an execution state of a ladder program in a debug system for the ladder program executed by a PC (programmable controller).

CONSTITUTION: The ladder program 1a to be generated is stored in a memory 1. When an execution condition designation means 2 designates an execution start position and an execution stop condition of the ladder program 1a, an execution stop control means 3 gives a command of starting the execution of the ladder program from the execution start position to a ladder execution means 4 and gives a command of stopping the execution of the ladder program to the ladder execution means 4 when the execution of the ladder program is coincident with the execution stop condition. And, the execution state when the execution is stopped, is displayed on a screen of a display device 6 under the control of a display control means 5.