In an engine cover mounting structure, an internal portion of an elastic member comprises a small diameter hole portion having a small inside diameter and a large diameter hole portion having a large inside diameter which continues to the small diameter hole portion, the relationship between an outside diameter W1 of a head portion which engages with the elastic member and an inside diameter W2 of the small diameter hole portion is set to be W1>W2, and the relationship between an inside diameter W3 of the large diameter hole portion and an outside diameter W4 of a leg portion is set to be W3.gtoreq.W4, the large diameter hole portion and the small diameter hole portion are made to communicate with each other in a step-like fashion, and at least part of a portion to be restrained is provided on an outer circumferential side of the large diameter hole portion.