To provide a vacuum pump mounting structure capable of mounting a vacuum pump to a narrow space between cam shafts without degrading the pump performance of the vacuum pump, in the vacuum mounting structure mounting the vacuum pump to a space at an engine front end portion wherein a difference in level is generated by mounting a phase variable mechanism.

If a boss portion 30 is formed to have the same diameter D as an outer peripheral edge 17L of a plate board 17A, the diameter D is larger than a side end portion 11D of a pump housing 11. Therefore, a shape of a small diameter portion 31 of the boss portion 30 is set to faucet fit in a segmental circular angle range removing the plate board 17 A part.