To provide an attaching structure of a stop plug capable of securely attaching the stop plug to an opening without causing damage to the stop plug, at the opening in an engine mounted on a vehicle or the like.

In the attaching structure of the stop plug for attaching the roughly columnar stop plug to the opening formed across both members on a joining surface where the first member and the second member join each other, the opening is formed of a first opening provided through the first member and a second opening provided through the second member, the axis of the opening passes the first opening, and the inner diameter of the second opening is larger than that of the first opening.