The present invention is for providing a milling cutter which can provide for both an improvement in sharpness and a retention of cutting edge strength, while making the transverse cross-section an easily manufactured polygon shape. Each optical assembly includes a first optical element and a second optical element. An inverter apparatus inverts the print media after the entrance roller directs the print media in the first direction and before the exit roller receives the print media. A length of a path traced by the magnetic flux in the ferromagnetic member is less than or equal to 1. 65 V and the no-load cell voltage response is monitored at predetermined intervals. A correcting section is provided to correct for a count taken at the count section, by use of collection efficiency data determined according to the gamma-ray detecting section and an energy the gamma rays possesses.