An image reading apparatus accurately determines the positions of image forming mirrors formed with an off-axis reflecting surface, while enabling any thermal expansion caused by a rise in temperature due to a rise in the surrounding temperature to be absorbed. Each image forming mirror formed with an off-axis reflecting surface comprises respective and independent units for determining the positions in the longitudinal direction, in the lateral position, and in the direction orthogonal to the reflecting surface. Holes with which the longitudinal position determining units and the lateral position determining units engage have gaps in the directions orthogonal to the position determining directions to absorb the thermal expansion caused by a rise in the temperature of the image forming mirror when the position determining units engage with the holes. Determination of the position of the reflecting surfaces of the image forming mirrors is carried out by urging faces provided on the reflecting surface side against the housing.