In a hydraulic apparatus, a first rotating member and a second rotating member are provided so as to be relatively rotatable with a same central axis, a cam is provided on the first rotating member, pistons are arranged on the second rotating member so as to be opposed to the cam, the pistons are pressed against the cam to contact by compression coil springs, oil chambers of which volumes are expanded and contracted according to movements of the pistons are provided on the second rotating member, first and second fluid paths through which oil flows into and out from the oil chambers are provided, and a path switching device for switching an inflow direction and an outflow direction of the oil in the first and second fluid paths according to difference in pressure between the first and second fluid paths is provided.