The present invention discloses a method of synthesizing an aliphatic polymer having a ketone group in the main chain thereof, in which polyhydric alcohol as a raw material is polymerized in the presence of a catalyst, and a method of preparing a composition containing an aliphatic polymer having a ketone group in the main chain thereof, including such a process. The rotation of the driving roller is controlled based on the signal from the encoder, and \xd7\u22661 is satisfied where \u03b11 is a correction coefficient for a fluctuation in thickness of the endless belt caused by a belt winding angle on the driving roller, and \u03b12 is a correction coefficient for a fluctuation in the thickness on the driven roller. Each of the brackets includes a slanted guide surface for guiding corresponding first hinge axle, wherein the pair of the first hinge axles are closer to a front portion of the cover than the second hinge axles.