To provide an erroneous assembly preventing structure for grommets, capable of positively preventing the grommets used in commonon right and left from being erroneously assembled.

With fitting parts 10 of right and left grommets 6, their shapes on both sides in the direction crossing a wire harness covering cylindrical body 7 at a place in circumferential direction of the cylindrical body are made to be asymmetrical. When the right and left grommets 6 are positioned in the same direction, the fitting part 10 of the right side grommet 1b is set to a state where it is turned 180 degrees in relation to the fitting 10 of the left side grommet 6a, and markings 12 with which the right and left grommets 6 can be distinguished from each other with respect to their position-direction or whether it is for left side assembly or for right side assembly, on the like, when the grommets are assembled, are provided on the right and left grommets 6. Further, fitting holes 8 into which the fitting parts 10 of the right and left grommets 6 are to be fitted at right and left car body panels 1, are provided on right and left of the car body in a state where they are turned 180 degrees from each other in response to the fitting parts 10 of the right and left grommets 6.