To provide an inexpensive handy-type grommet mounting device having simple structure, capable of easily expanding/contracting the grommet, and safely and quickly mounting a grommet on a bundle of wire harness with small number of processes.

The grommet mounting device is composed of at least a vertical housing and an expanding/contracting arm expansion/contraction operation structure. A wire harness insertion window and a wire harness insertion/pull-out notched part are arranged on the vertical housing. Four pieces of the expanding/contracting arm expansion/contraction operation structure are extended in radial direction from central part of the insertion window of the vertical housing, and respective expanding/contracting arm expansion/contraction operation structure mount the expanding/contracting arms on a holding arm in free rotation/stopping movement. The holding arm is jointed to a piston rod of an air cylinder mechanism and a guide rod of a guide rod mechanism. The air cylinder mechanism is mounted on the vertical housing, and the guide rod mechanism is fixed to the vertical housing, and the expanding/contracting arms are made to locate at middle part of the insertion window.