To provide a plural-beam scanning optical device for an electrophotographic device capable of facilitating the adjustment of optical path, preventing the optical path from being adjusted in a state where it is deviated from a specified optical path and obtaining stable optical performance.

A direction where a single laser is split into plural lines in a light splitting element 3 is regulated to be on a plane 21 nearly parallel with a scanning plane 20, and a beam modulating direction in an AO modulator 4 is regulated to be on a plane 22 nearly perpendicular to the scanning plate, and a roof prism 7 is arranged so that the maximum plane 7a other than the incident and emitting surfaces of the prism may be 0° or about 90° to the scanning plane.