Provided is a backup device, etc. The controller may also be configured to perform storing processes for storing detection results of the first sensor and the second sensor in the memory when the controller determines that the length of the recording medium is less than or equal to the conveying distance. In a preferred arrangement, the two strings of opposed parallel coupled diodes are used with different numbers of diodes in each string. A plurality of amplitude values among the emphasized amplitude values are corrected as corrected amplitude values at some of frequencies within the frequency band. With a controller appropriately controlling timings to apply an accelerating voltage to the accelerating electrode tubes, accelerating energy can be gained each time the charged particle passes through gaps between the accelerating electrode tubes.