An image reading device includes first and second reading units, first and second converting units, and a setting unit. The first and second reading units include N-number of first sub-regions and M-number of second sub-regions, respectively. Each converting unit includes L-number of channels. Each channel converts analog signal into digital signal. When the setting unit sets the double-sided read mode, P-number of first sub-regions are connected to the first converting unit and M-number of second sub-regions are connected to the second converting unit. When the read mode setting unit sets the single-sided read mode, P-number of first sub-regions are connected to the first converting unit and the remaining first sub-regions are connected to the second converting unit. L is greater than or equal to M and smaller than or equal to N. N is smaller than or equal to 2×L.