Hydraulic pump state monitoring device and hydraulic drive system

A hydraulic drive system comprising:

• a prime mover,
• a tank that stores a hydraulic operating fluid,
• a variable displacement hydraulic pump driven by rotation of the prime mover to suck the hydraulic operating fluid in the tank and deliver the hydraulic operating fluid with capacity according to a tilting amount, and
• a controller that acquires information relating to a rotational force of the prime mover, the tilting amount of the hydraulic pump, and delivery pressure of the hydraulic operating fluid concerning the hydraulic pump and controls the tilting amount of the hydraulic pump, wherein
• the hydraulic drive system comprises a pressure sensor that senses the delivery pressure of the hydraulic pump, and
• the controller is configured to compute a feature amount relating to force conversion efficiency that is efficiency of conversion from the rotational force of the prime mover to the delivery pressure of the hydraulic operating fluid concerning the hydraulic pump, in a case in which the controller has controlled the tilting amount of the hydraulic pump to a predetermined tilting amount and the delivery pressure of the hydraulic pump sensed by the pressure sensor has become a predetermined pressure.