An imaging system includes an imaging apparatus which is provided with an imaging device and a mount adaptor which adjusts a flange focal length between the imaging apparatus and an interchangeable lens, wherein the mount adaptor is provided with a phase difference focus detection section which performs focus detection from a phase difference and an optical device which splits incident light from the interchangeable lens into incident light of the imaging device and incident light of the phase difference focus detection section, and the imaging apparatus is provided with a contrast focus detection section which performs focus detection from contrast using the imaging device, an adjustment section which adjusts the focus position of the phase difference focus detection section, and an adjustment control section which activates adjustment control of the focus position using detection of the mounting of the mount adaptor and the interchangeable lens.