An insertion auxiliary implement of the present invention includes: a tubular part into which a flexible endoscope insertion part which is insertable into a body cavity, and one of a treatment tool and a channel into which the treatment tool is insertable, are insertable; and a sealing member which has through holes for supporting the endoscope insertion part and one of the treatment tool and the channel in the tubular part, and which airtightly and movably contacts each of a periphery of the endoscope insertion part, a periphery of the treatment tool or a periphery of the channel, and an inner surface of the tubular part, and thereby maintains airtightness between a distal end and a proximal end inside the tubular part.