A method of computing a conveyance variance from a difference of two adjustment patterns in order to measure a variance in a conveyance amount occurring while a sheet is conveyed. By using this method, the variance that occurs during one rotation of the roller occurring because of the roller accuracy, deflection of the roller, and the attachment of a roller supporting member can be alleviated. Thus, the unevenness occurring synchronously to one rotation of the roller can be alleviated, and as a result, an ink jet printer that is capable of printing with high quality can be provided.