PURPOSE: To surely monitor the congestion execution state, congestion clear state and abnormal congestion state of a program and to appropriately grasp the execution state of the program.

CONSTITUTION: This program execution monitoring device for a controller for executing a program in accordance with a starting period is provided with a starting frequency counter to be started up at the time of starting the program and turned down at the end of program execution, a congestion state judging means 23A for judging a congestion state from the count value of the starting frequency counter and a congestion limit set value, a congestion flag updating means 23C for setting up a congestion flag when the flag is not stored in a congestion flag memory at the time of judging a congestion state at present and clearing the congestion flag when the preceding congestion flag is stored in the memory at the time of judging no congestion at present and a congestion informing means 23D for reporting the generation of congestion at the time of setting up the congestion flag and reporting congestion clear at the time of clearing the flag.