A diagnosing device is provided for a vehicle having a flow passage for circulating a liquid medium for cooling an inverter of the vehicle, and a pump provided on the flow passage to circulate the liquid medium. The diagnosing device includes a temperature sensor for measuring a temperature related to a temperature of the liquid medium, and a calculating unit for calculating a concentration of the liquid medium. The calculating unit calculates a viscosity of the liquid medium based on a flow rate of the liquid medium flowing through the flow passage and performance of the pump, and when the calculation as to the viscosity of the liquid medium is executed with a plurality temperature conditions, the calculating unit calculates the concentration of the liquid medium based on a relationship between the plurality of temperature conditions and the corresponding viscosities of the liquid medium.