An introduction assistance instrument with a side hole (11) includes a tubular body (13) adapted to be inserted into a tubular organ of a human body. A side hole (21) is formed in a peripheral wall of the tubular body (13) so as to allow a medical instrument to be introduced into the tubular organ through the side hole (21) of the tubular body (13). Further, there is provided at a tip portion of the tubular body (13) a backflow restriction mechanism configured to restrict backflow of a body fluid in the tubular organ from a tip opening of the tubular body (13) to the side hole (21). The backflow restriction mechanism may be a non-return valve provided between the tip opening of the tubular body (13) and the side hole (21) in an inner lumen of the tubular body (13) or a tip member provided at the tip of the tubular body (13) and having a through hole formed at a central portion thereof.
