An optical scanning unit includes a light source including a plurality of light-emitting elements; a light detector to detect a light beam emitted from the light source; a light-flux splitter, angled to the optical axis of the light beam emitted from the light source, having an aperture, a portion of reduced thickness susceptible to warping, a concave face of the warped light-flux splitter as a reflecting face, and a convex face of the warped light-flux splitter opposite the concave face as a non-reflecting face; and a light-flux splitter pressing unit to press the light-flux splitter onto a light-flux splitter holding member without blocking the aperture. Light beam passed the aperture is used as a write-use light flux. The reflecting face reflects a light flux other than the write-use light flux as a monitor-use light flux. The light-flux splitter pressing unit presses a portion of maximum convexity of the non-reflecting face.