The device for treating working fluid includes a fluid level detecting device for detecting a fluid level of cleaned fluid in a cleaned fluid tank, and a control unit for determining an adjustment value for a discharge rate command for a filter device pump, based on the fluid level detected by the fluid level detecting device. The control unit operates to determine a renewed discharge rate command by adding the adjustment value thus determined to the present discharge rate command, and to output the renewed discharge rate command to a pump controller. The pump controller drives the filter device pump according to the discharge rate command to transfer the working fluid from the polluted fluid tank to the cleaned fluid tank through the filter device.