Disclosed is a head of an optical data recording and/or reproducing device. The head is provided with a laser source, an objective optical system, a deflector for deflecting the laser beam emitted by the laser source towards the objective optical system. A relay optical system provided between the deflector and the objective optical system. Further, a detector is provided between the deflector and the relay optical system. The detector has a first and a second light receiving portions and an opening. The two light receiving portions are arranged in a first direction in which the laser beam is deflected by the deflector. A part of the laser beam incident on the detector is received by the first and second light receiving portions, the other passes through the opening. An amount of rotation of the deflector is detected in accordance with amounts of light received by the two light receiving portions.