A method of correcting unevenness in scanning lines in a light beam scanning apparatus having a mechanical scanning light beam deflector which rotates about a rotational axis to deflect a scanning light beam in a main scanning direction and a correcting light beam deflector for deflecting the scanning light beam in a sub-scanning direction, comprising steps of continuously detecting deflection of the rotational axis of the scanning light beam deflector to obtain nonperiodic deviation of the scanning light beam in the sub-scanning direction, storing the difference between a position of the scanning light beam detected at a starting point immediately before the effective scanning range and the nonperiodic deviation of the scanning light beam as a starting point deviation, storing the difference between a position of the scanning light beam detected at an end point immediately behind the effective scanning range and the nonperiodic deviation of the scanning light beam as an end point deviation, successively calculating deviation of the scanning light beam from a target scanning position in the middle of straight scanning on the basis of the stored starting point deviation, the stored end point deviation and the rotating angle of the scanning light beam deflector as periodic deviation, and determining the amount by which the scanning light beam is to be deflected in the sub-scanning direction by the correcting light beam deflector on the basis of the sum of the nonperiodic deviation and the periodic deviation.