A thermostat device provided with a cylindrical boss protruding into a flow of fluid flowing in from a fluid inlet inside a device housing from a direction obstructing the flow of fluid, wherein a rectifying wall in the shape of a thin plate that protrudes toward an upstream side of the flow of fluid from the boss is provided as a rectifying means. The rectifying wall is formed in the shape of a plate that gradually increases in thickness from the fluid inlet side toward the boss. The leading edge along a direction of the flow of fluid is formed to have a tapered shape inclined from the tip of the boss toward the base of the boss. A side of the boss opposite the side facing the fluid inlet is provided with a rib.