A water injection apparatus for a water jet loom, includes a weft insertion nozzle for weft insertion of a weft yarn by injection water, a weft insertion pump pumping water into the weft insertion nozzle, a water storage tank storing water introduced into the weft insertion pump, and a suction pipe passing water from the water storage tank into the weft insertion pump. The water injection apparatus includes an auxiliary water supply unit that is provided separately from the weft insertion pump and can supply water to the suction pipe. At least when the weft insertion pump sucks water, the auxiliary water supply unit supplies water to the suction pipe.