To provide a photoelectric separation-type sensor where a cover fitted to a main body case can easily be opened/closed without interrupting the adjusting work of a main body.

In the smoke sensor 100, a light emitting device and a light receiving device are separately arranged and the presence or absence of smoke between the devices is detected. Each of the casing of the light emitting device and the light receiving device is constituted of a main body case 20 where a main body 30 including a light emitting means or the light receiving means is stored and a front cover 10 fitted to the front side of the main body cases 20 so as to cover the main body. The main body 30 emits or receives light through the front cover 10. The front cover shafts 21 and 21 formed in such a way that they are almost orthogonal with the forward/backward direction of the main body case 20 are installed at the upper parts of the main body case 20. Shaft holes 14a and 14a engaged with the shafts 21 and 21 are installed in the front cover 10 and the front cover 10 is fitted so that the shaft holes 14a and 14a are engaged with the shafts 21 and 21.