A thermostat apparatus may include a thermostat case formed of a first passage and a second passage, a valve body disposed in the thermostat case, wherein a first valve may be formed at one end side thereof to open or close the first passage, a retainer, one end portion of which may be mounted on the other end side of the valve body, wherein a second valve may be formed at the other end portion of the retainer to open or close the second passage, an elastic member that elastically biases the retainer in a direction to make the first valve close the first passage and make the second valve open the second passage, and a drive portion that may be configured to push the retainer in a direction that according to temperature, the first valve opens the first passage and the second valve closes the second passage.