To provide a highly waterproofing grommet with ease of installation on a vehicle body panel which is even applicable to high pressure while ensuring high waterproofing.

This highly waterproofing grommet 40 consists of a grommet body 41 formed out of an elastic material and attached to a through-hole 62, a pressing member 51 which presses the grommet body 41 against an installation panel P at the time of installation, and hooks 61, 61 for engaging the pressing members 51 in the vicinity of the through-hole 62 of the installation panel P. The pressing member 51 comprises a plate member 52 for transmitting a pressing force to the grommet body 41 roughly equally, and a spring lock 55 which is engaged with the hooks 61, 61 and presses the grommet body 41 against the mounting panel P along with the plate member 52. The grommet body 41 comprises a latching rib 44 for latching the pressing members 51. A holding parts 54 for integrally holding the plate member 52, and the spring lock 55 are provided on the plate member 52.