A first signal processing apparatus, to which a first endoscope apparatus including a first image sensor is detachably attached and communicably connected with a second signal processing apparatus to which a second endoscope apparatus including a second image sensor is attached, the first signal processing apparatus being configured to process a first imaging signal generated by the first image sensor or a second image signal generated by the second image sensor, includes a control unit configured to control a process inside the first signal processing apparatus in accordance with communication with the second signal processing apparatus. The control unit processes the first imaging signal when a first command is received, and when a second command is received, the control unit controls the process inside the first signal processing apparatus to make the process of the second imaging signal to correspond to the second command.