To make a fire sensor inexpensive, easy to install, apparently thin, and nice-looking.

This sensor 1 is provided with a base 2 in which a cylindrical part 20 having a top plate is inserted into a mounting hole formed at a ceiling plate, and a corner-shaped part 20b formed at the peripheral edge of the lower opening is mounted so as to be abutted to the ceiling plate at the peripheral edge of the mounting hole, and a head 3 engaged in the base from the lower opening so as to be freely attachable and detachable. In this case, this sensor 1 is provided with a tie plate spring 25 extended from the side face of the cylindrical part 2 to almost the diametral direction viewed on a plane so as to be made bendable from the neighborhood of a root 25a to almost the upper direction viewed from the side face within an elastic limit, and non-slip working is performed on the ceiling plate abutted part of a flange part.