An endoscope system includes: an insertion part inserted into a body; an observation part provided at the insertion part and observing an inside of a viewing range defined in the front thereof; an arm part manipulated to be bent, a proximal end portion of which is provided at the insertion part, and which has an opening of a channel, through which a treatment tool is inserted, provided at a distal end side thereof; a first detection part detecting an out-of-viewing-range state where a center axis line in the opening of the channel does not pass through the viewing range; a second detection part detecting that the treatment tool has been inserted through the channel; and an alert part alerting a user when the second detection part detects that the treatment tool has been inserted through the channel and the first detection part detects the out-of-viewing-range state.