According to an embodiment of this invention, opening/closing of an ADF unit is correctly detected even under the influence of outside light. More specifically, a light source provided in a reading unit is turned on to irradiate a white board, and first data based on reflected light from the white board is obtained. Next, the light source provided in the reading unit is turned off, and second data based on reflected light from the white board is obtained. The difference between the first data and second data is compared with a predetermined threshold. It is determined based on the comparison whether the ADF unit is in an opened or closed state.