Disclosed is an image reader including a first reading unit that reads an image on a front surface of an object placed on a first background member to output a first output value; a second reading unit that reads an image on a back surface of the object placed on a second background member which is different from the first background member to output a second output value; a ground output detecting unit that detects first and second ground output values based on the first and second output values, respectively; a thickness calculating unit that calculates the thickness of the object based on an output ratio between the first ground output value and the second ground output value; and an output correction unit that corrects the first and second output values based on the calculated thickness of the object.