A light scanning unit comprising: a light source to emit a light beam according to an image signal; a light deflector to deflect and scan the light beam emitted from the light source in a main scanning direction; a synchronization sensor (160, 360) comprising a light-receiving surface (161, 361) to receive a portion of the light beam deflected and scanned from the light deflector to generate a synchronization signal of the light scanning unit, wherein the light-receiving surface comprises a front side and an end side with respect to an advancing path of the light beam in the main scanning direction; and at least one blocking member (170, 470, 471, 472) disposed at the end side of the light-receiving surface so as to shield a part of the light receiving surface to define an effective light-receiving surface whose width in the main scanning direction varies according to a sub scanning direction; wherein a boundary of the front side of the effective light-receiving surface is orthogonal to the main scanning direction of the light beam, wherein the at least one blocking member is arranged such that the effective light receiving surface is symmetrically formed in a sub scanning direction with respect to a central line of an advancing path of the light beam in the main scanning direction, wherein the boundary of the front side of the effective light-receiving surface of the synchronization sensor with respect to the advancing direction of the light beam in the main scanning direction is a straight line, and wherein a boundary of an end side of the effective light-receiving surface of the synchronization sensor with respect to the advancing direction of the light beam in the main scanning direction is stepped, or wherein the at least one blocking member comprises a first slanted blocking member part positioned at a first side of the light receiving surface and a second slanted blocking member part positioned at a second side of the light receiving surface opposite to the first side, and wherein the first slanted blocking member part and the second slanted blocking member part are spaced apart from each other in the sub scanning direction such that a portion of the end of the effective light receiving surface is orthogonal to the main scanning direction of the light beam. a light source to emit a light beam according to an image signal; a light deflector to deflect and scan the light beam emitted from the light source in a main scanning direction; a synchronization sensor (160, 360) comprising a light-receiving surface (161, 361) to receive a portion of the light beam deflected and scanned from the light deflector to generate a synchronization signal of the light scanning unit, wherein the light-receiving surface comprises a front side and an end side with respect to an advancing path of the light beam in the main scanning direction; and at least one blocking member (170, 470, 471, 472) disposed at the end side of the light-receiving surface so as to shield a part of the light receiving surface to define an effective light-receiving surface whose width in the main scanning direction varies according to a sub scanning direction; wherein a boundary of the front side of the effective light-receiving surface is orthogonal to the main scanning direction of the light beam, wherein the at least one blocking member is arranged such that the effective light receiving surface is symmetrically formed in a sub scanning direction with respect to a central line of an advancing path of the light beam in the main scanning direction, wherein the boundary of the front side of the effective light-receiving surface of the synchronization sensor with respect to the advancing direction of the light beam in the main scanning direction is a straight line, and wherein a boundary of an end side of the effective light-receiving surface of the synchronization sensor with respect to the advancing direction of the light beam in the main scanning direction is stepped, or wherein the at least one blocking member comprises a first slanted blocking member part positioned at a first side of the light receiving surface and a second slanted blocking member part positioned at a second side of the light receiving surface opposite to the first side, and wherein the first slanted blocking member part and the second slanted blocking member part are spaced apart from each other in the sub scanning direction such that a portion of the end of the effective light receiving surface is orthogonal to the main scanning direction of the light beam.