An image sensor includes a pixel unit having first and second photoelectric converters, an amplifier provided commonly for the first and second photoelectric converters, first and second transfer transistors configured to respectively transfer charges generated in the first and second electric converters to an input portion of the amplifier. The signal read out by the readout portion includes a first optical signal read out in a state in which charges are transferred from the first photoelectric converter to the input portion by the first transfer transistor, and a second optical signal read out, after the readout of the first optical signal, in a state in which charges are transferred from the second photoelectric converter to the input portion by the second transfer transistor.