A fastening structure comprising: an annular washer that is disposed adjacent to a plate-shaped resin member at one plate thickness direction side of the resin member and that includes a washer main body fabricated of metal; a bolt fabricated of metal, including: a head portion that is disposed at an opposite side of the washer from a side thereof at which the resin member is disposed and that, together with the washer, sandwiches a fastened member, and a threaded portion that projects from the head portion to another plate thickness direction side of the resin member and that penetrates through an inside of the washer and an inside of a penetrating hole formed in the resin member; a nut fabricated of metal that is screwed onto the threaded portion; a collar including a collar main body fabricated of metal that includes: a tubular portion through which the threaded portion penetrates, and a flange that is formed at an outer periphery portion of the tubular portion and that is disposed at the other plate thickness direction side of the resin member, the collar main body touching against the washer main body and the nut; characterized in that the fastening structure further comprises: a gap regulation portion configured to regulate a gap between the flange and the resin member, wherein the gap regulation portion includes: a base portion fabricated of resin that is configured to structure a portion of the collar and that is disposed between the flange and the resin member, the base portion being structured integrally with the flange, a retaining member configured to retain the base portion at the resin member, wherein: the retaining member is provided at an opposing face of the base portion that opposes the resin member, and the retaining member is an adhesive member that is compressed between the base portion and the resin member. an annular washer that is disposed adjacent to a plate-shaped resin member at one plate thickness direction side of the resin member and that includes a washer main body fabricated of metal; a bolt fabricated of metal, including: a head portion that is disposed at an opposite side of the washer from a side thereof at which the resin member is disposed and that, together with the washer, sandwiches a fastened member, and a threaded portion that projects from the head portion to another plate thickness direction side of the resin member and that penetrates through an inside of the washer and an inside of a penetrating hole formed in the resin member; a head portion that is disposed at an opposite side of the washer from a side thereof at which the resin member is disposed and that, together with the washer, sandwiches a fastened member, and a threaded portion that projects from the head portion to another plate thickness direction side of the resin member and that penetrates through an inside of the washer and an inside of a penetrating hole formed in the resin member; a nut fabricated of metal that is screwed onto the threaded portion; a collar including a collar main body fabricated of metal that includes: a tubular portion through which the threaded portion penetrates, and a flange that is formed at an outer periphery portion of the tubular portion and that is disposed at the other plate thickness direction side of the resin member, the collar main body touching against the washer main body and the nut; a tubular portion through which the threaded portion penetrates, and a flange that is formed at an outer periphery portion of the tubular portion and that is disposed at the other plate thickness direction side of the resin member, the collar main body touching against the washer main body and the nut; characterized in that the fastening structure further comprises: a gap regulation portion configured to regulate a gap between the flange and the resin member, wherein the gap regulation portion includes: a base portion fabricated of resin that is configured to structure a portion of the collar and that is disposed between the flange and the resin member, the base portion being structured integrally with the flange, a retaining member configured to retain the base portion at the resin member, wherein: the retaining member is provided at an opposing face of the base portion that opposes the resin member, and the retaining member is an adhesive member that is compressed between the base portion and the resin member. a gap regulation portion configured to regulate a gap between the flange and the resin member, wherein the gap regulation portion includes: a base portion fabricated of resin that is configured to structure a portion of the collar and that is disposed between the flange and the resin member, the base portion being structured integrally with the flange, a retaining member configured to retain the base portion at the resin member, wherein: a base portion fabricated of resin that is configured to structure a portion of the collar and that is disposed between the flange and the resin member, the base portion being structured integrally with the flange, a retaining member configured to retain the base portion at the resin member, wherein: the retaining member is provided at an opposing face of the base portion that opposes the resin member, and the retaining member is an adhesive member that is compressed between the base portion and the resin member.