1. A multibeam laser control device for an image forming apparatus comprising: a control unit to preprocess image data in parallel using plural preprocessing units and output preprocessing data accumulated in a buffer to a multibeam generating unit; a signal interpolating unit to send out an interpolation signal which period is shifted from a beam detection signal sent out by detecting a beam of the multibeam generating unit; and a signal processing unit to send out an image processing signal obtained by ORing the beam detection signal and the interpolation signal.