An image reader of the present disclosure includes a contact glass, an automatic document feeder, a first reading module, and a second reading module. The first and second reading modules each acquire black reference data through reading performed by a sensor with a light source turned off, and acquires white reference data through reading performed by the sensor at a position facing a white reference plate with the light source turned on. In a case of performing continuous one-side reading by means of the first reading module, based on a sensor output value of the second reading module in the continuous one-side reading, it is determined whether it is necessary for the first reading module to reacquire the black reference data or the white reference data.