A light detecting method and apparatus wherein non-projection imaging operation of a predetermined range is carried out once within a non-projection imaging operation time thereby to obtain background information. During a remaining time after the non-projection imaging operation, light is radiated. At least two imaging operations of a predetermined range are performed thereby to acquire projection information. Information on the radiated light is extracted based on the background information and the projection information. Accordingly, the dead time that may occur before extraction of reflected light can be suppressed.