An apparatus includes a pixel array in which pixels for outputting an analog signal are arranged in a matrix, vertical output lines each of which is connected to pixels in a same column, A/D conversion units, which are individually connected to the vertical output lines, for converting the analog signal into a digital signal, and a constant current supply unit for supplying a constant current to the A/D conversion units. Each of the A/D conversion units includes an integration unit for integrating the constant current, a comparison unit for comparing the integrated constant current with the analog signal and outputting a comparison signal, and a digital signal storage unit for storing a digital signal corresponding to the comparison signal. The integration unit includes an input capacitor for receiving the constant current. The comparison unit is connected to the constant current supply unit via the input capacitor.