An optical scanning unit comprising: a light source for emitting light according to an image signal; a deflector for deflection-scanning a luminous flux emitted from the light source; an incident optical system for causing the luminous flux emitted from the light source to be incident on the deflector; and an image forming optical system for imaging the luminous flux deflected by the deflector on an image-formed surface, wherein the incident optical system comprises a luminous flux limit device limiting the luminous flux, the luminous flux limit device comprising: a body portion including an aperture; and a light-blocking portion to prevent light from being transmitted via a portion of the aperture, wherein the aperture comprises: a blocking region in which light is blocked by the light-blocking portion, and a transmissive region (383,384) which is a remaining region of the aperture excluding the blocking region; wherein the blocking region is located in a central region of the aperture, the transmissive region (383,384) surrounds the blocking region; and the transmissive region (383,384) comprises a first transmissive region and a second transmissive region, the phases of the luminous fluxes of which are set to be different, the first transmissive region surrounds the blocking region, and the second transmissive region surrounds the first transmissive region; characterized in that if a radius of an outermost portion of the blocking region in a main scanning direction is ρ 1m, a radius of an outermost portion of the first transmissive region in the main scanning direction is ρ 2m, and a radius of an outermost portion of the second transmissive region in the main scanning direction is ρ 3m, then ρ 1m <ρ 2m <ρ 3m, 0.3≤ρ 1m /ρ 3m ≤0.7, and 0.5≤ρ 2m /ρ 3m ≤0.9 are satisfied. a light source for emitting light according to an image signal; a deflector for deflection-scanning a luminous flux emitted from the light source; an incident optical system for causing the luminous flux emitted from the light source to be incident on the deflector; and an image forming optical system for imaging the luminous flux deflected by the deflector on an image-formed surface, wherein the incident optical system comprises a luminous flux limit device limiting the luminous flux, the luminous flux limit device comprising: a body portion including an aperture; and a light-blocking portion to prevent light from being transmitted via a portion of the aperture, wherein the aperture comprises: a blocking region in which light is blocked by the light-blocking portion, and a transmissive region (383,384) which is a remaining region of the aperture excluding the blocking region; wherein the blocking region is located in a central region of the aperture, the transmissive region (383,384) surrounds the blocking region; and the transmissive region (383,384) comprises a first transmissive region and a second transmissive region, the phases of the luminous fluxes of which are set to be different, the first transmissive region surrounds the blocking region, and the second transmissive region surrounds the first transmissive region; characterized in that if a radius of an outermost portion of the blocking region in a main scanning direction is ρ 1m, a radius of an outermost portion of the first transmissive region in the main scanning direction is ρ 2m, and a radius of an outermost portion of the second transmissive region in the main scanning direction is ρ 3m, then ρ 1m <ρ 2m <ρ 3m, 0.3≤ρ 1m /ρ 3m ≤0.7, and 0.5≤ρ 2m /ρ 3m ≤0.9 are satisfied. a body portion including an aperture; and a light-blocking portion to prevent light from being transmitted via a portion of the aperture, wherein the aperture comprises: a blocking region in which light is blocked by the light-blocking portion, and a transmissive region (383,384) which is a remaining region of the aperture excluding the blocking region; a blocking region in which light is blocked by the light-blocking portion, and a transmissive region (383,384) which is a remaining region of the aperture excluding the blocking region; wherein the blocking region is located in a central region of the aperture, the transmissive region (383,384) surrounds the blocking region; and the transmissive region (383,384) comprises a first transmissive region and a second transmissive region, the phases of the luminous fluxes of which are set to be different, the first transmissive region surrounds the blocking region, and the second transmissive region surrounds the first transmissive region; characterized in that if a radius of an outermost portion of the blocking region in a main scanning direction is ρ 1m, a radius of an outermost portion of the first transmissive region in the main scanning direction is ρ 2m, and a radius of an outermost portion of the second transmissive region in the main scanning direction is ρ 3m, then ρ 1m <ρ 2m <ρ 3m, 0.3≤ρ 1m /ρ 3m ≤0.7, and 0.5≤ρ 2m /ρ 3m ≤0.9 are satisfied. ρ 1m <ρ 2m <ρ 3m, 0.3≤ρ 1m /ρ 3m ≤0.7, and 0.5≤ρ 2m /ρ 3m ≤0.9