To provide a semiconductor device technique shortening a processing time of Si anisotropic wet etching for forming a bridge structure by removing an Si (100) substrate itself under a membrane part and beam parts.

The membrane part 51 and the beam parts 52-55 are formed in the <100> direction of the Si (100) substrate, and the beam parts 52-55 support the membrane part 51 only on two facing sides of the membrane part 51, and a length of the shortest part of the beam parts 52-55 is constituted to be longer than the width. A heat-sensitive part 90 (in the case of an infrared sensor) for detecting an infrared ray and an infrared absorption film 91 are formed on the membrane part 51, and a signal generated by the detected infrared ray is guided from the heat-sensitive part 90 to the outside through the membrane part 51 and the beam parts 53, 55 by wires 92, 93. The membrane part 51 is protected from etching solution by a bottom surface protection layer 94, an interlayer film 95 and an upper surface protection layer 96.