To provide a temperature presuming device which can correctly presume a temperature of hydraulic system without using a temperature sensor.

The device can presume the temperature without using the temperature sensor through the following steps: a relation between a pressure and a temperature in an enclosed gas within a bladder of accumulator is memorized; a change rate switching between the fluid supply capacity and the pressure is determined based on a fluid supply capacity by an oil pump and a pressure detected by a pressure sensor; a switching point pressure at which this change rate is switched is obtained (step S14); and the temperature of the enclosed gas within the bladder is presumed by the switching point pressure (step S16) based on the relation between the pressure and the temperature of the enclosed gas.