An object of the present invention is to detect the home position with a high accuracy without being affected by temperature. The present invention is an image reading apparatus having a reading portion, and the apparatus includes: an acquisition unit configured to acquire first data obtained by the reading portion reading a main scanning line in a state where a light source of the reading portion is turned on in a first light quantity, and second data obtained by the reading portion reading the main scanning line in a state where the light source of the reading portion is turned on in a second light quantity smaller than the first light quantity; a generation unit configured to generate difference data by subtracting the value of the second data from the value of the first data; and a determination unit configured to determine whether a mark arranged at a position corresponding to a home position of the reading portion has been detected on the main scanning line based on the difference data.