According to the present invention, in a cooling water control valve disposed in a cooling water circuit of a water-cooled engine, switching of the cooling water flowing into and out of a heat-accumulating tank and control of an amount of the cooling water passing through a bypass water passage are performed by a single control valve element. Further, a bypass water passage is opened according to a decrease of an absolute of intake negative pressure detected by a pressure sensor and is closed according to the increase of the absolute of the intake negative pressure.