An image reading apparatus comprising: a detection mechanism configured to detect a position of a conveyed medium; a background color switching mechanism (93, 93A) arranged on an opposite side of the conveyed medium from the detection mechanism, and configured to switch a background color of the conveyed medium detected by the detection mechanism; and a light transmissive member arranged between the detection mechanism and the background color switching mechanism (93, 93A), wherein the conveyed medium is conveyed between the light transmissive member and the background color switching mechanism (93, 93A) while being in contact with the light transmissive member, the light transmissive member is provided with two indicators (406L, 406R) indicating a reference position, and the detection mechanism is configured to calculate the position of the conveyed medium in a main scanning direction based on position information detected from the reference position, wherein the background color switching mechanism (93, 93A) includes background portions in a plurality of colors provided to change a circumferential surface facing the detection mechanism, and the circumferential surface includes a shading surface, a conveying surface (931a to 934a), a surface for correcting a magnification error deviation in a main scanning direction, and a gray balance adjustment surface, wherein when the detection mechanism reads the indicator of the light transmissive member, the background color switching mechanism (93, 93A) performs driving and controlling so that the shading surface becomes a background colour of the conveyed medium, and the detection mechanism reads the indicators (406L, 406R) with a shading control being turned off, wherein the surface for correcting the magnification error deviation includes a glass scale in which a scale is formed by a group of multiple longitudinal lines arranged along the main scanning direction of the detection mechanism, and in the magnification error deviation correction mode, the detection mechanism corrects, based on information detected from the scale of the glass scale, the position information detected from the reference position, and when the distance between the indicators (406L, 406R) read by the detection mechanism changes from a reference value with respect to the glass scale and then calculates, on the basis of position information about the indicators (406R, 406L), a magnification error correction value to cancel the change so that the distance is corrected using the magnification error correction value. a detection mechanism configured to detect a position of a conveyed medium; a background color switching mechanism (93, 93A) arranged on an opposite side of the conveyed medium from the detection mechanism, and configured to switch a background color of the conveyed medium detected by the detection mechanism; and a light transmissive member arranged between the detection mechanism and the background color switching mechanism (93, 93A), wherein the conveyed medium is conveyed between the light transmissive member and the background color switching mechanism (93, 93A) while being in contact with the light transmissive member, the light transmissive member is provided with two indicators (406L, 406R) indicating a reference position, and the detection mechanism is configured to calculate the position of the conveyed medium in a main scanning direction based on position information detected from the reference position, wherein the background color switching mechanism (93, 93A) includes background portions in a plurality of colors provided to change a circumferential surface facing the detection mechanism, and the circumferential surface includes a shading surface, a conveying surface (931a to 934a), a surface for correcting a magnification error deviation in a main scanning direction, and a gray balance adjustment surface, wherein when the detection mechanism reads the indicator of the light transmissive member, the background color switching mechanism (93, 93A) performs driving and controlling so that the shading surface becomes a background colour of the conveyed medium, and the detection mechanism reads the indicators (406L, 406R) with a shading control being turned off, wherein the surface for correcting the magnification error deviation includes a glass scale in which a scale is formed by a group of multiple longitudinal lines arranged along the main scanning direction of the detection mechanism, and in the magnification error deviation correction mode, the detection mechanism corrects, based on information detected from the scale of the glass scale, the position information detected from the reference position, and when the distance between the indicators (406L, 406R) read by the detection mechanism changes from a reference value with respect to the glass scale and then calculates, on the basis of position information about the indicators (406R, 406L), a magnification error correction value to cancel the change so that the distance is corrected using the magnification error correction value.