A transfer device for an imaging apparatus, comprising: a transfer roller to transfer an image formed on an image carrier to a transfer medium in the imaging apparatus, the transfer roller including a rotation shaft comprising a conductive material and an ion conductive member disposed around the rotation shaft; a power source to generate a transfer voltage; and a switch device to selectively connect, during a rotation of the transfer roller, a power-supply path from the power source to the transfer roller among a plurality of power-supply paths based on whether or not the image is being transferred from the image carrier, to reverse the direction of an electric field applied by the transfer voltage to the ion conductive member, characterised in that the transfer roller is movable to contact the image carrier and to be spaced apart from the image carrier; and the switch device is to select the power-supply path: by moving the transfer roller to contact the image carrier in order to set the power-supply path to extend from the power source, via the rotation shaft of the transfer roller, via the ion conductive member of the transfer roller, via the image carrier and to an electric ground, when the image is being transferred, and by moving the transfer roller away from the image carrier in order to set the power-supply path to extend from the power source, via the ion conductive member of the transfer roller, via the rotation shaft of the transfer roller, and to the electric ground, when the image is not being transferred. a transfer roller to transfer an image formed on an image carrier to a transfer medium in the imaging apparatus, the transfer roller including a rotation shaft comprising a conductive material and an ion conductive member disposed around the rotation shaft; a power source to generate a transfer voltage; and a switch device to selectively connect, during a rotation of the transfer roller, a power-supply path from the power source to the transfer roller among a plurality of power-supply paths based on whether or not the image is being transferred from the image carrier, to reverse the direction of an electric field applied by the transfer voltage to the ion conductive member, characterised in that the transfer roller is movable to contact the image carrier and to be spaced apart from the image carrier; and the switch device is to select the power-supply path: by moving the transfer roller to contact the image carrier in order to set the power-supply path to extend from the power source, via the rotation shaft of the transfer roller, via the ion conductive member of the transfer roller, via the image carrier and to an electric ground, when the image is being transferred, and by moving the transfer roller away from the image carrier in order to set the power-supply path to extend from the power source, via the ion conductive member of the transfer roller, via the rotation shaft of the transfer roller, and to the electric ground, when the image is not being transferred. by moving the transfer roller to contact the image carrier in order to set the power-supply path to extend from the power source, via the rotation shaft of the transfer roller, via the ion conductive member of the transfer roller, via the image carrier and to an electric ground, when the image is being transferred, and by moving the transfer roller away from the image carrier in order to set the power-supply path to extend from the power source, via the ion conductive member of the transfer roller, via the rotation shaft of the transfer roller, and to the electric ground, when the image is not being transferred.