An adapter for an endoscope, the adapter including: a cylindrical body having an internal portion formed on the cylindrical body, the internal portion configured to matingly engage with each of distal and proximal external portions provided on a tip of the endoscope, the distal and proximal external portions being separated in a longitudinal direction of the cylindrical body by a portion of the tip that does not engage the internal portion; and wherein the cylindrical body further having an anti-slip portion formed on an exterior of the cylindrical body, the anti-slip portion having a plurality of recesses arranged circumferentially on the exterior, a position of the anti-slip portion in the longitudinal direction being set such that a position of the proximal external portion in the longitudinal direction does not overlap with the longitudinal position of the anti slip portion where the proximal external portion is engaged with the internal portion.