To solve a problem associated with a device for reacting substances in a microspace such as a microdevice wherein accurate measurement of the temperature of a fluid flowing in its passage is required for the purpose of controlling the reaction temperature, and wherein the fluid flow is disturbed if its temperature sensor is disposed in the passage and the temperature in the passage cannot accurately be measured if the temperature sensor is disposed outside the passage.

The passage forming body (1) equipped with a temperature sensor includes a seal metal-fitting (11), a temperature sensor (12) with a temperature measuring part (37) thereof bonded to the seal metal-fitting (11), and a heat-insulation member (13) that covers the seal metal-fitting (11) and the temperature sensor (12), whereby the heat generated in a passage (21) can accurately be detected without disturbing the fluid flow.