An image forming apparatus comprising: a light source configured to emit a light beam; a reflecting mechanism configured to reflect said light beam such that an irradiation point of the light beam moves in a predetermined scanning direction; a light receiving element that is arranged in a movement plane defined by the light beam reflected by the reflecting mechanism, and configured to output a light reception signal for adjusting an irradiation timing of the light beam in accordance with reception of the light beam reflected by the reflecting mechanism; and a restricting mechanism configured to position the light receiving element relative to the movement plane, a control board configured to control the light source in accordance with the light reception signal, wherein the restricting mechanism includes a holding plate (514A, 514B, 514C), the holding plate (514A, 514B, 514C) having a holding portion configured to hold the light source, and wherein the light receiving element is mounted to the control board, and the control board is connected to the holding plate (514A, 514B, 514C) characterized in that said holding plate (514A, 514B, 514C) comprises a first notch face (557, 558) that defines a first notch portion that crosses the movement plane, the light receiving element contacts the first notch face (557,558), wherein a through hole is formed in the control board, the holding plate (514A, 514B, 514C) includes a boss that is configured to be inserted into the through hole formed in the control board, and the first notch portion is formed such that the light receiving element is configured to be inserted into the first notch portion when the control board is rotated about the boss. a light source configured to emit a light beam; a reflecting mechanism configured to reflect said light beam such that an irradiation point of the light beam moves in a predetermined scanning direction; a light receiving element that is arranged in a movement plane defined by the light beam reflected by the reflecting mechanism, and configured to output a light reception signal for adjusting an irradiation timing of the light beam in accordance with reception of the light beam reflected by the reflecting mechanism; and a restricting mechanism configured to position the light receiving element relative to the movement plane, a control board configured to control the light source in accordance with the light reception signal, wherein the restricting mechanism includes a holding plate (514A, 514B, 514C), the holding plate (514A, 514B, 514C) having a holding portion configured to hold the light source, and wherein the light receiving element is mounted to the control board, and the control board is connected to the holding plate (514A, 514B, 514C) characterized in that said holding plate (514A, 514B, 514C) comprises a first notch face (557, 558) that defines a first notch portion that crosses the movement plane, the light receiving element contacts the first notch face (557,558), wherein a through hole is formed in the control board, the holding plate (514A, 514B, 514C) includes a boss that is configured to be inserted into the through hole formed in the control board, and the first notch portion is formed such that the light receiving element is configured to be inserted into the first notch portion when the control board is rotated about the boss.