A signal processing device includes: a connection detector that detects whether an endoscope including one or more operation members is connected a signal selector that selects an operation state signal that is input from the endoscope when it is detected that the endoscope is connected, and that selects an operation state signal that is input from an external input device including one or more operation members that correspond to the one or more operation members of the endoscope when it is detected that the endoscope is not connected an allocation information storage unit that stores allocation information relating to an operation that is allocated to each of the operation members an operation instruction setting unit that sets an operation instruction according to the operation state signal selected by the signal selector in accordance with the operation state signal selected by the signal selector and the allocation information and an operation execution unit that executes an operation according to the operation instruction set by the operation instruction setting unit.
