A thermostat device that controls a flow rate of a liquid coolant in a cooling system circuit, the thermostat device comprising: a cylindrical valve seat; a main valve which closes the cylindrical valve seat by being caused to approach the inside of the cylindrical valve seat; and a thermosensitive movable body which drives the main valve by detecting a temperature of a liquid coolant, wherein an outer circumference of an approach portion to the cylindrical valve seat of the main valve is provided with a circular elastic body that is elastically pressed against an inner peripheral surface of the cylindrical valve seat when the valve is closed continuously throughout the whole circumference thereof, and the inner peripheral surface of the cylindrical valve seat is sealed by the circular elastic body throughout the whole circumference thereof when the valve is closed, an outer peripheral surface of the circular elastic body is provided with a circular sealing lip protruding outwardly from the outer peripheral surface in a circumferential direction in a lip-like manner, the circular sealing lip is provided so as to form a waveform having concavity and convex portion curving in a drive direction of the main valve and to be displaced in a circumferential direction, due to the thermosensitive movable body, and the liquid coolant starts to flow in and out through a liquid coolant passage formed between the cylindrical valve seat and a convex portion of the circular sealing lip when the main valve moves from a valve closing state, and a curving convex portion of the circular sealing lip is separated from the inner peripheral surface of the cylindrical valve seat. a cylindrical valve seat; a main valve which closes the cylindrical valve seat by being caused to approach the inside of the cylindrical valve seat; and a thermosensitive movable body which drives the main valve by detecting a temperature of a liquid coolant, wherein an outer circumference of an approach portion to the cylindrical valve seat of the main valve is provided with a circular elastic body that is elastically pressed against an inner peripheral surface of the cylindrical valve seat when the valve is closed continuously throughout the whole circumference thereof, and the inner peripheral surface of the cylindrical valve seat is sealed by the circular elastic body throughout the whole circumference thereof when the valve is closed, an outer peripheral surface of the circular elastic body is provided with a circular sealing lip protruding outwardly from the outer peripheral surface in a circumferential direction in a lip-like manner, the circular sealing lip is provided so as to form a waveform having concavity and convex portion curving in a drive direction of the main valve and to be displaced in a circumferential direction, due to the thermosensitive movable body, and the liquid coolant starts to flow in and out through a liquid coolant passage formed between the cylindrical valve seat and a convex portion of the circular sealing lip when the main valve moves from a valve closing state, and a curving convex portion of the circular sealing lip is separated from the inner peripheral surface of the cylindrical valve seat.