A medium conveying apparatus includes a conveying roller to convey a medium, a first unit, a second unit located to face the first unit, an imaging sensor provided in the first unit, to image the medium conveyed by the conveying roller, a reference member including a facing surface, and provided in the second unit in such a way that the facing surface is rotatable between a facing position at which the facing surface faces the imaging sensor, and a non-facing position at which the facing surface deviates from the facing position, a guide portion to slidably guide the second unit in a direction perpendicular to a medium conveying surface so that the second unit moves in the direction perpendicular to the medium conveying surface, and a support portion to rotatably support the reference member. The support portion is located inside the guide portion.