To detect actual deterioration from digital pulse signals output from an encoder in which the wave period, duty cycle, phase difference and phase direction vary according to the operating speed, resolution and movement direction and are difficult to determine, which makes it difficult to detect deterioration, and to eliminate the wasted time and expense for the maintenance involved in taking the necessary measures. A system and a method for monitoring output signals of an encoder, in which an output signal output from an encoder body 1 corresponding to a basic operation pattern of an object 20 being measured which is set in an initial state is associated with the basic operation pattern and stored in a storage device 30; the object 20 being measured is then operated in accordance with the basic operation pattern at fixed intervals; an output signal output from the encoder body 1 at that time and output signal data in the initial state stored in the storage device 30 are compared, and a warning is output if t
