One of the tools to measure the surface temperature of an object without contact is the radiation thermometer that measures the dose of infrared rays emitted from the surface of the object. HORIBA's radiation thermometer, available in various types to meet a wide range of applications, were demanded to improve their response time in applications to measurement of moving objects or observation of rapid temperature changes. The responce time of the radiation thermometer is dependent on the response time of the infrared sensor installed inside the thermometer. We developed, as an infrared sensor, the thermopile with a unique heat sensing element using the MEMS (Micro-Electro-Mechanical Systems) technology, which mainly uses a silicon semiconductor processing technique to produce a microstructure. Further, we optimized the sensor structure, optical system and signal processing system and successfully realized the fastest response time as a radiation thermometer that can measure temperatures below the room temperature.