A printing device includes a first roller for rotatably retaining a roll body where a medium is rolled, a first motor for rotating the first roller, a second roller installed further to the downstream side of the roll body in the feeding direction of the medium to feed the medium in the feeding direction or in a reverse feeding direction which is opposite to the feeding direction corresponding to the rotating direction, a second motor for rotating the second roller, and a controller which causes the medium to become slack the first roller and the second roller by feeding the medium by a predetermined feeding amount in the reverse feeding direction by the second motor, and then rotates the roll body in a winding-up direction by the first motor so that the slackness decreases to calculate the diameter of the roll body based on the rotation amount of the first roller at that time and the predetermined feeding amount.