Transfer-conveyance device and method capable of controlling transfer bias according to change in environmental condition

A transfer-conveyance device for transferring a visible image on an image bearing member in an image forming apparatus onto a transfer sheet and to convey the transfer sheet. The transfer-conveyance device includes a contact member that contacts the image bearing member, a bias applying device to apply a transfer bias to the contact member, a resistance detecting device to detect a resistance of the contact member, and a transfer bias control device to determine an amount of variation in the resistance of the contact member and control the transfer bias. The transfer bias is controlled on the basis of the amount of variation in the resistance of the contact member. A method of controlling a transfer bias in an image forming apparatus includes the steps of applying the transfer bias to a contact member contacting an image bearing member to transfer an image on the image bearing member onto a transfer sheet, determining an amount of variation in a resistance of the transfer member, and controlling the transfer bias on the basis of the amount of variation in the resistance of the contact member.