An image reading apparatus including: two image reading devices which are different from each other, read out a same surface of a same sheet after image formation on a sheet conveyance path and read out a plurality of common color patches formed in the same surface of the same sheet, one of the image reading devices being a first image reading device which reads out only a partial region in a main scanning direction and the other of the image reading devices being a second image reading device which reads out over an image formation width in the main scanning direction; and a calculation section which estimates a value equivalent to reading information of the first image reading device from reading information of the second image reading device on the basis of reading information obtained by reading out the common color patches.