An endoscope accessory, used with an endoscope including a suction conduit communicating with a suction device, an accessory insertion channel, and an operation portion at which entrances of the conduit and channel are formed, includes a tubular insertion unit to be inserted into the channel through the entrance of the channel and to be movable along the channel, and a tubular connection unit provided at a proximal end of the insertion unit and detachably connected to the entrance of the suction conduit. An endoscope system is constituted by combining the accessory with the endoscope. In a method for mounting the accessory onto the endoscope, the insertion unit of the accessory is inserted into the channel through its entrance in the operation portion of the endoscope to be movable along the channel, and the connection unit of the accessory is detachably connected to the entrance of the conduit.