A change computing arrangement computing an amount of change in a measured value of a measurement signal of a raindrop sensor in a raindrop quantity sensing execution time period that is a time period, during which a wiper blade moves outside of a sensing range of the raindrop sensor. A difference computing arrangement computes a difference between a predetermined reference value and an initial measured value of the measurement signal of the raindrop sensor. A determining arrangement determines the quantity of raindrops on the windshield based on the amount of change, which is computed by the change computing arrangement, and the difference, which is computed by the difference computing arrangement.