1. A surgical assistant system comprising: an operation unit configured to give an input; an arm unit on which a treatment tool is mounted; an operation imaging unit configured to acquire an operation image that is an image including the operation unit; an endoscope configured to acquire an internal-body image including an image of an interior of a body of a patient; a mode control unit having a plurality of operation modes and configured to enable one of the plurality of operation modes to be set as a setting operation mode; a driving unit configured to enable the arm unit to be operated based on the input given by the operation unit and the setting operation mode; a synthetic image creation unit configured to synthesize images using at least the operation image to create a synthetic image based on the setting operation mode; and a display unit configured to display the synthetic image.