To provide a beam interval measuring device, capable of measuring with high accuracy a beam interval in any one direction of the beam interval in the main scanning direction in two light beams and the beam interval in the sub-scanning direction in a simple structure.

A photodiode 20 comprises a slit rectangular light receiving region. A straight line 20a on a scan start side of this light receiving region is arranged aslant by θs (0°<θs<90°) with respect to the subscanning direction. A CPU computes a beam interval distance ΔY12, based on a time lag Δt between the time of penetration of a laser beam LB2 into the straight line 20a and the time of penetration of a laser beam LB1 into the straight line 20a in the same scan period, an inter-beam distance ΔX12 in the main scanning direction, a scan speed V, and the setting angle θs of the straight line 20a with respect to the sub-scanning direction.