An on-vehicle generator mounting device includes: a stay provided with a first insertion hole; a front side mounting leg and a rear side mounting leg of the generator, both being disposed on two ends of the stay; a second insertion hole provided through one of the mounting legs; a cylindrical bush fitted movably into a fitting hole provided through the other of the mounting legs; a clamp nut disposed at the other mounting leg to be adjacent to and coaxially with the bush, so as not to turn within said aperture but with a gap in a radial direction therebetween; and a bolt inserted through the second insertion hole, first insertion hole and bush, the bolt being screw-engaged with the clamp nut. An end of the clamp nut protrudes outward of said aperture beyond an outer surface of the other mounting leg. The clamp nut screw-engaged with the bolt fixing the generator is prevented from corroding due to anti-freezing agent.