A thermostat device having a thermal expansion unit that expands with a temperature increase and contracts with a temperature decrease, the thermal expansion unit built into an element case so as to be susceptible to effects of heat from outside the element case, the thermostat device having: a piston disposed inside the element case along an axial direction of the element case that advances and retreats with expansion and contraction of the thermal expansion unit, having an inner end that is immersed in the thermal expansion unit and an outer end that protrudes from an opening in one end of the element case; a guide member that is inserted into the element case from the opening in one end of the element case and slidably holds the piston; a seal member that is inserted into the element case from the opening in one end of the element case and holds the thermal expansion unit so that the thermal expansion unit does not leak from a chamber in which it is contained; and a first valve fitted onto the exterior of the element case near the opening in one end of the element case wherein the first valve opens and closes a flowpath, wherein a portion of the element case for fitting the first valve onto the exterior of the element case, and portions of the element case for inserting the guide member and the seal member, are disposed so as to be offset in an axial direction and in a diametrical direction of the element case, a diameter of a portion of the element case that guides the first valve onto the element case is configured to be smaller than an outer diameter of the element case where the first valve is fitted onto the element case, and an inner diameter of the element case where the guide member and the seal member are inserted into the element case is configured to be smaller than an inner diameter of the element case where the first valve is fitted onto the element case, and the guide member is configured to be insertable in the element case without contacting an inner wall of the element case where the first valve is fitted onto the element case. a piston disposed inside the element case along an axial direction of the element case that advances and retreats with expansion and contraction of the thermal expansion unit, having an inner end that is immersed in the thermal expansion unit and an outer end that protrudes from an opening in one end of the element case; a guide member that is inserted into the element case from the opening in one end of the element case and slidably holds the piston; a seal member that is inserted into the element case from the opening in one end of the element case and holds the thermal expansion unit so that the thermal expansion unit does not leak from a chamber in which it is contained; and a first valve fitted onto the exterior of the element case near the opening in one end of the element case wherein the first valve opens and closes a flowpath, wherein a portion of the element case for fitting the first valve onto the exterior of the element case, and portions of the element case for inserting the guide member and the seal member, are disposed so as to be offset in an axial direction and in a diametrical direction of the element case, a diameter of a portion of the element case that guides the first valve onto the element case is configured to be smaller than an outer diameter of the element case where the first valve is fitted onto the element case, and an inner diameter of the element case where the guide member and the seal member are inserted into the element case is configured to be smaller than an inner diameter of the element case where the first valve is fitted onto the element case, and the guide member is configured to be insertable in the element case without contacting an inner wall of the element case where the first valve is fitted onto the element case.