To provide a detector the detecting block which can be easily assembled with a printed board.

In a detector which is provided with a detector main body 2 in which a printed board 4 having engaging holes 4a are incorporated so that the holes 4a can be faced to the outside, a detecting section 5 which detects a prescribed physical quantity, and a detecting block 3 which has a supporting frame 6 for supporting the detecting section 5 and in which the main body 2 and the block 3 are assembled together by engaging ratchets 6b having thin front end sections 6c and thick base end sections 6d and provided at the front ends of extension pieces 6a extended from the frame 6 with the engaging holes 4a, ratchet pieces 6g which are elastically deformed independently from the pieces 6a are formed by forming notched sections 6f into the base end sections 6d.