0 A light beam deflecting apparatus which measures the passing time of a light beam in reciprocating scans on a specified position on the scanning line, and then calculates time lag data corresponding to the time lag in the reciprocating scan of a light beam deflecting device with respect to the drive signal therefor. From a measurement of two passing times in forward and backward scans, timing data is calculated, corresponding to the modulation start or end time of the light beam within a scanning cycle period. The invention resolves the drawback, in a light beam deflecting device for displaying an image by reciprocating light beam scannings, of image variation by variations for example in amplitude, phase and offset.