An image reading device, at a starting time point, starts one mode gain setting processing, which is processing of obtaining a gain set value of a shared amplifier in one of a color mode and a monochrome mode. After the one mode gain setting processing is completed, the image reading device performs an other mode gain setting processing, which is processing of obtaining a gain set value of the shared amplifier in the other mode. In a case where an instruction to perform a reading job in the one mode is received while the other mode gain setting processing is being performed, the image reading device discontinues the other mode gain setting processing and performs the reading job in the one mode.