An optical writing apparatus comprising: a deflecting part having a deflection plane for deflecting a plurality of light beams; a synchronization detection mirror for reflecting the plurality of light beams (L1, L2) from the deflecting part; a synchronization detection part for detecting the timing for deflecting the plural light beams (L1, L2) reflected from the synchronization detection mirror, the synchronization detection part including a lens for converging the plural light beams (L1, L2) reflected from the synchronization detection mirror, a light detection plate for detecting the plural light beams (L1, L2) converged by the lens, and a holding member for holding the lens and the light detection plate; and a housing for housing the deflecting part and the synchronization detection part, characterized in that the synchronization detection mirror is downwardly inclined at a predetermined angle with respect to a plane perpendicular to a scanning plane of the deflecting part, wherein the synchronization detection mirror reflects the plural light beams to the lens at a downward acute angle with respect to the scanning plane of the deflecting part, wherein the holding member has a lens holding plane that is inclined in a sub-scanning direction to hold the lens in an upward inclined state, wherein the lens holding plane is inclined substantially perpendicular with respect to the plural light beams incident on the lens, wherein the holding member has an opening and a space that are formed at a substantially center portion thereof between lens and the light detection plate for inserting a fastening member therethrough, wherein both sides of the holding member are formed by walls for encasing the space therein, and wherein the holding member has a hole at a bottom thereof continuing from the space for fastening the fastening member to the housing. a deflecting part having a deflection plane for deflecting a plurality of light beams; a synchronization detection mirror for reflecting the plurality of light beams (L1, L2) from the deflecting part; a synchronization detection part for detecting the timing for deflecting the plural light beams (L1, L2) reflected from the synchronization detection mirror, the synchronization detection part including a lens for converging the plural light beams (L1, L2) reflected from the synchronization detection mirror, a light detection plate for detecting the plural light beams (L1, L2) converged by the lens, and a holding member for holding the lens and the light detection plate; and a housing for housing the deflecting part and the synchronization detection part, characterized in that the synchronization detection mirror is downwardly inclined at a predetermined angle with respect to a plane perpendicular to a scanning plane of the deflecting part, wherein the synchronization detection mirror reflects the plural light beams to the lens at a downward acute angle with respect to the scanning plane of the deflecting part, wherein the holding member has a lens holding plane that is inclined in a sub-scanning direction to hold the lens in an upward inclined state, wherein the lens holding plane is inclined substantially perpendicular with respect to the plural light beams incident on the lens, wherein the holding member has an opening and a space that are formed at a substantially center portion thereof between lens and the light detection plate for inserting a fastening member therethrough, wherein both sides of the holding member are formed by walls for encasing the space therein, and wherein the holding member has a hole at a bottom thereof continuing from the space for fastening the fastening member to the housing.