An adapter for use with an endoscope. The adapter including: at least first and second axis-shifted lenses, each of the first and second axis-shifted lenses including a truncated portion, the first and second axis shifted lenses being arranged side by side so that respective first and second optical axes are parallel to each other； and a prism arranged further distally than the first and second axis-shifted lenses, the prism being configured to change a viewing direction of each of the first and second axis-shifted lenses； wherein at least a portion of each of the truncated portions of the first and second axis-shifted lenses is arranged further outward in the radial direction than the prism.
