A photoelectric sensor comprises a control unit which performs a measuring process for data on an amount of light input from a light-receiving unit. The control unit measures the length TPS of period (from point a to point d) from when measurement data starts to change in the direction of a threshold P0 to when the amount of received light returns to the level at which the measurement data started to change, and the length TON of period (from point b to point c) for which the detection signal is ON-level. The control unit then outputs information on a margin for TPS with respect to TON on a display unit provided for the photoelectric sensor or to an external device in order to display them.