A detection apparatus determines whether an output of the light-receiving unit meets a predetermined reference level when the light emission amount of the light-emitting unit is changed, under a condition that the output of the light-receiving unit changes according to a change in a light emission amount of the light-emitting unit. The detection apparatus detects, based on the output from the light-receiving unit, that the light emitted from the light-emitting unit to the light-receiving unit is blocked, under a condition that the light-emitting unit is emitting light based on a light emission amount emitted from the light-emitting unit when it has been determined that the output of the light-receiving unit meets the predetermined reference level.