PURPOSE: To provide a method and a device to display a degree of stability by simultaneously monitoring an input converting signal or a synchronizing signal from an optical encoder.

CONSTITUTION: A monitoring signal frequency 50 is reversed by an inverter 52, and is inputted to an AND gate 56. A time reference generator 54 is connected to the AND gate 56. The time reference generator 54 outputs a signal having a specific frequency to the AND gate 56. Outputs of the AND gate 56 are inputted to a first counter 58, and are added up. The monitoring signal frequency is inputted to a second counter 62, and all the pulses having a signal frequency are added up. A clock circuit 64 capable of outputting a reset signal is arranged so that the second counter 62 is reset after a prescribed time passes. The clock circuit 64 is connected to a latch circuit 66, and the latch circuit 66 is connected to a comparator 68 to which an inspection table 60 is connected. The comparator 68 is connected to a display unit 70 through a connector 72.