An image forming apparatus includes: a transfer unit, a reflectance sensor, a time-series data acquiring unit, and a transfer-unit rotation-time calculating unit. The time-series data acquiring unit is configured to acquire time-series data with respect to the image carrier background-portion reflectance, measured by the reflectance sensor, as reference data over a time period twice longer than a time period that by design the image carrier requires for one cycle, and as advance/delay data over a time period twice longer than a time period longer or shorter in multiples of a predetermined time interval than the by-design time period. The transfer-unit rotation-time calculating unit is configured to calculate a one-cycle-time calculation value that is the time actually required for one cycle of the image carrier, based on the reference data and the advance/delay data acquired by the time-series data acquiring unit.