An insertion assisting device includes a rotary body in the form of toroid, and a support and drive part for supporting the rotary body in a rotatable manner and rotating it. The support and drive part has an approximately cylindrical supporting member for supporting the rotary body in a rotatable manner. The rotary body is provided with a convex portion for increasing the thickness to improve the strength thereof. A concave portion having a shape corresponding to the shape of the convex portion is formed on a portion of a front end surface of the supporting member, which is made in contact with the convex portion. The concave portion prevents the rotary body from stopping rotating, which may be caused by point contact between the convex portion and the supporting member.