Device and method for heating

PURPOSE:To improve the welding quality by controlling the surface temperature of the object to be machined to the prescribed temperature based on the intensity ratio of radiant light beams in the same machining range with the object to be machined. CONSTITUTION:Radiant light beams of the same machining range with the object to be machined 4 are collected during machining, the collected radiant light beams are divided to plural numbers, the radiant light beam of respectively different wave length of the divided radiant light beams is made to pass through filters 10, 11, the intensity ratio of each light beam is picked by detecting the light strength of different wave length passed through the filters and the surface temperature of the object to be machined is researched based on the strength ratio. The surface temperature of the object to be machined is controlled to the prescribed temperature based on the above surface temperature. Therefore, the welding or the surface reforming with excellent quality can be executed.