Abstract: A detector with 180.degree. detecting range, including a base, a housing, a detecting lens, a drive circuit, a sensor and a signal deflector, wherein said base is L-shaped and located at a detecting position, a hole being formed on the base, a projection being formed on the housing corresponding to the hole of the base, the projection being fitted in the hole, permitting the housing to be rotated and adjusted, a semicircular detecting opening being formed at front end of the housing, the detecting lens being disposed in the detecting opening, a drive circuit being disposed in the housing, the sensor being fitted in the signal deflector and mounted on a base board of the drive circuit, a signal receiving opening being formed in front of the signal deflector to aim at the sensor, by means of the signal deflector, a signal within dead corner over 120.degree. range being deflected toward the signal receiving opening so that the detecting range of the detector is enlarged to 180.degree..