The device relates to a small-sized photoelectric smoke detector in which the measuring chamber itself has a light trap function and which has a narrow measuring space. The measuring chamber is composed of a front wall (6) which reflects the light from a light emitter (1) at a predetermined angle to the optical axis of the smoke detector, side walls (4, 5) which are arranged nearly parallel with the optical axis. According to a preferred embodiment of the device the measuring chamber comprises a rear wall (7) which reflects the light reflected from the side walls (4, 5) at such an angle that the reflected light does not directly irradiate the light receiver (2). The light trap function of the measuring chamber can be further improved if the walls of the measuring chamber have mirror-like surfaces with good absorbance.
