PURPOSE: To perform frequent play diagnoses and improve measurement precision by vibrating a hydraulic servo system with sine waves at the frequency in which the servo system can follow, judging the presence of the play of a machine system, and calculating the quantity of the play.

CONSTITUTION: A play diagnostic device 37 inputs a sine wave signal (a) at the frequency in which a servo system 23 can sufficiently follow to the hydraulic servo system 23 from a sine wave oscillation section 38 to vibrate it. Detected signals at a servo system spool position (g) and a machine system position (h) are sampled in a fixed period and stored in sampling sections 40, 42. Stored data are analyzed by an analysis section 44, and the play quantity is calculated by an arithmetic section 46. Frequent play diagnoses can be performed, and measurement precision is improved.