An assist device which is applied to a treatment instrument including an insertion body to be inserted through a channel of an endoscope, and which is configured to assist movement of the insertion body relative to the channel, includes : a flexible tube including a passage into which the insertion body of the treatment instrument is inserted a connector provided on the flexible tube and configured to connect to the endoscope to allow communication between the channel and the passage a first retainer provided on the flexible tube and configured to hold an outer circumference of an insertion section of the endoscope or an outer circumference of an operation section connected to the insertion section and a second retainer being movable relative to the first retainer and configured to hold the insertion body of the treatment instrument.
