Abstract: Disclosed is a device for detecting an incident position of radiation beam, particularly, its center or center of gravity. The detecting device is provided with a scanning type radiation beam sensing device having a plurality of radiation sensing elements in a linear arrangement, and the sensing device is disposed in such a manner that its radiation receiving surface may be substantially coincided with an incident surface of the radiation beam to be detected. When reading an output from the sensing device, the time sequential output signals from the sensing device are split into predetermined sections, and the signal quantities among the sections are compared. In this way, the position of the center or the center of gravity of the radiation beam on the incident surface is detected with the position corresponding to a split point of the signals as the reference.