Image forming apparatus and image forming method for controlling a secondary transfer bias according to recording sheet type

An image forming apparatus includes an image bearer to bear a toner image, a nip forming member contacting the image bearer to form a transfer nip, and a transfer power source. The transfer bias has a transfer peak value to electrostatically move toner from the image bearer to the recording sheet and an opposite-peak value to electrostatically move less toner from the image bearer to the recording sheet than the transfer peak value. An opposite-peak duty of the transfer bias is less than 50%. The opposite-peak duty is tr/T×100% where T is one cycle of the transfer bias and tr is a time period during which the transfer bias is on a side of the opposite-peak value relative to an offset voltage of the transfer bias in the one cycle T.