A register panel mounting structure includes: a retainer having a cylindrical shape and having a flow path formed therein to permit passage of air for air conditioning; and a register panel mounted at an edge portion of the retainer, the edge portion forming an opening at a downstream side. The register panel has a shape only covering, from a front side, one portion of the edge portion in a circumferential direction. The register panel includes: an inner panel disposed at a side at which the opening is located when viewed from the edge portion; and an outer panel disposed opposite to the side at which the opening is located when viewed from the edge portion. The inner panel and the outer panel sandwich one portion of the edge portion in the circumferential direction.