An image forming apparatus includes a transfer belt to carry a toner image, a transfer roller to make contact with the transfer belt to transfer the toner image onto a sheet from the transfer belt, an opposite roller facing the transfer roller across the transfer belt, a current application unit to apply a predetermined current to the transfer roller or opposite roller when there is no sheet at a transfer position, a voltage detection unit to detect a voltage generated at a transfer position by applying the predetermined current to the transfer roller or opposite roller by the current application unit, a sheet sensor to detect a kind of the sheet, and a humidity sensor to detect intra-apparatus humidity. The image forming apparatus further includes a transfer bias voltage application unit to calculate a transfer roller correction voltage on the basis of the voltage detected by the voltage detection unit, calculate a sheet correction voltage on the basis of the sheet kind detected by the sheet sensor and the intra-apparatus humidity detected by the humidity sensor, add the calculated transfer roller correction voltage and sheet correction voltage, thereby decide a transfer bias voltage, and apply the decided transfer bias voltage to either of the transfer roller and the opposite roller so as to transfer the toner image onto the sheet from the transfer belt.