A method of diagnosing a hydraulic machine which comprises a rotor shaft, a cylinder, a piston forming a working chamber together with the cylinder, and a high-pressure valve and a low-pressure valve provided for the working chamber, and which is configured to perform conversion between rotational motion of the rotor shaft and reciprocal motion of the piston, the method comprising: a pressure-measuring step of detecting a pressure of the working chamber; and a damage-detecting step of detecting a damage of the low-pressure valve on the basis of a pressure detection result obtained in the pressure-measuring step, wherein the low-pressure valve is a poppet valve which is disposed between a low-pressure line of the hydraulic machine and the working chamber, and which is for controlling a communicating state between the low-pressure line and the working chamber, wherein the damage-detecting step comprises detecting the damage of the low-pressure valve on the basis of a change rate of a pressure of the working chamber in each cycle of the reciprocal motion of the piston, characterised in that, the damage-detecting step comprises detecting the damage of the low-pressure valve on the basis of an index obtained by dividing the maximum value of the change rate of the pressure of the working chamber, in each cycle of the reciprocal motion of the piston, by a speed of the piston at the time when the change rate of the pressure of the working chamber is at the maximum value, and wherein the index is a normalized value represented by an expression: dP / dt max × Vref/V* where (dP/dt) max is the maximum value of the change rate of the pressure of the working chamber, V ref is a speed of the piston at a predetermined position, and V* is the speed of the piston at the time when the change rate of the pressure of the working chamber is at the maximum value (dP/dt) max. a pressure-measuring step of detecting a pressure of the working chamber; and a damage-detecting step of detecting a damage of the low-pressure valve on the basis of a pressure detection result obtained in the pressure-measuring step, wherein the low-pressure valve is a poppet valve which is disposed between a low-pressure line of the hydraulic machine and the working chamber, and which is for controlling a communicating state between the low-pressure line and the working chamber, wherein the damage-detecting step comprises detecting the damage of the low-pressure valve on the basis of a change rate of a pressure of the working chamber in each cycle of the reciprocal motion of the piston, characterised in that, the damage-detecting step comprises detecting the damage of the low-pressure valve on the basis of an index obtained by dividing the maximum value of the change rate of the pressure of the working chamber, in each cycle of the reciprocal motion of the piston, by a speed of the piston at the time when the change rate of the pressure of the working chamber is at the maximum value, and wherein the index is a normalized value represented by an expression: dP / dt max × Vref/V* where (dP/dt) max is the maximum value of the change rate of the pressure of the working chamber, V ref is a speed of the piston at a predetermined position, and V* is the speed of the piston at the time when the change rate of the pressure of the working chamber is at the maximum value (dP/dt) max.