An image forming apparatus includes: a latent image forming unit forming a latent image; a transfer body on which a reference index for setting an output start time point of image data is formed; a detecting unit facing the reference index and outputting a detection signal changing in accordance with passing of the reference index; a measuring unit measuring change duration time; and a controller controlling the output start time point with the detection signal. The controller starts a first period during which change of the detection signal is ignored, according to first change of the detection signal, starts a second period after the first period, regards change of the detection signal occurring first in the second period as a reference of the output start time point if the change duration time is longer than a predetermined period, and ignores change of the detection signal after the change occurring first.