PURPOSE: To provide an encoder diagnostic device in which an absolute value encoder or incremental encoder can be easily diagnosed.

CONSTITUTION: The absolute value data transmitted from an absolute value encoder 12 when a power source is OFF is stored in a nonvolatile memory 25. When the power source is then ON, the above absolute value data stored in the nonvolatile memory 25 is compared with the absolute valve data transmitted from the absolute value encoder at that time, and when the difference is out of a predetermined allowable range, an alarm is displayed on an indicator 26. The data stored in the nonvolatile memory 25 may be the count value stored in an INC counter 23. In operation, the absolute value data provided from the absolute value encoder 12 is compared with the count value of the INC counter 23 when the rotating speed of a motor 6 is less than a fixed value, and the alarm is displayed on the indicator 26 when the different is out of the predetermined allowable range.