A wiring harness installation structure comprising: a grommet fittingly mounted in a harness insertion hole, which is formed so as to penetrate through a body panel which separates an interior area and an exterior area of a vehicle compartment, for holding a wiring harness inserted through the harness insertion hole, and a protector formed of a hard resin material and having a cylindrical construction which accommodates the wiring harness for protection is equipped in a harness installation path in the exterior area of the vehicle compartment, characterised in that the grommet further includes: a panel fitting tubular portion which is fittingly held in the harness insertion hole and through which the wiring harness is inserted: and an extension tubular portion which is formed at an end portion of the panel fitting tubular portion in a side of the exterior area of the vehicle compartment so as to extend from the end portion while surrounding a periphery of the wiring harness; wherein the extension tubular portion has a required length to fittingly connect an end portion of the extension tubular portion to a harness inlet/outlet opening of the protector which is arranged adjacent to the grommet, thereby prevent an exposure of the wiring harness from the body panel to the protector and wherein the harness insertion hole is formed in the body panel at the position higher than an end portion of the protector to which the grommet is fitted, wherein the extension tubular portion is adapted to be deformed to match the installation path of the wiring harness wherein the extension tubular portion is configured as a combination of a straight tube-like normal tubular portion and a bellows tube-like flexible tubular portion, wherein the straight tube-like normal tubular portion being arranged between the bellows tube-like flexible tubular portion. a grommet fittingly mounted in a harness insertion hole, which is formed so as to penetrate through a body panel which separates an interior area and an exterior area of a vehicle compartment, for holding a wiring harness inserted through the harness insertion hole, and a protector formed of a hard resin material and having a cylindrical construction which accommodates the wiring harness for protection is equipped in a harness installation path in the exterior area of the vehicle compartment, characterised in that the grommet further includes: a panel fitting tubular portion which is fittingly held in the harness insertion hole and through which the wiring harness is inserted: and an extension tubular portion which is formed at an end portion of the panel fitting tubular portion in a side of the exterior area of the vehicle compartment so as to extend from the end portion while surrounding a periphery of the wiring harness; wherein the extension tubular portion has a required length to fittingly connect an end portion of the extension tubular portion to a harness inlet/outlet opening of the protector which is arranged adjacent to the grommet, thereby prevent an exposure of the wiring harness from the body panel to the protector and wherein the harness insertion hole is formed in the body panel at the position higher than an end portion of the protector to which the grommet is fitted, wherein the extension tubular portion is adapted to be deformed to match the installation path of the wiring harness wherein the extension tubular portion is configured as a combination of a straight tube-like normal tubular portion and a bellows tube-like flexible tubular portion, wherein the straight tube-like normal tubular portion being arranged between the bellows tube-like flexible tubular portion. a panel fitting tubular portion which is fittingly held in the harness insertion hole and through which the wiring harness is inserted: and an extension tubular portion which is formed at an end portion of the panel fitting tubular portion in a side of the exterior area of the vehicle compartment so as to extend from the end portion while surrounding a periphery of the wiring harness; wherein the extension tubular portion has a required length to fittingly connect an end portion of the extension tubular portion to a harness inlet/outlet opening of the protector which is arranged adjacent to the grommet, thereby prevent an exposure of the wiring harness from the body panel to the protector and wherein the harness insertion hole is formed in the body panel at the position higher than an end portion of the protector to which the grommet is fitted, wherein the extension tubular portion is adapted to be deformed to match the installation path of the wiring harness wherein the extension tubular portion is configured as a combination of a straight tube-like normal tubular portion and a bellows tube-like flexible tubular portion, wherein the straight tube-like normal tubular portion being arranged between the bellows tube-like flexible tubular portion.