To obtain a mold clamping apparatus for a vertical injection molding machine, etc., wherein the number of parts is less, piping work is simple, cost is cheap, a space for fitting a boost cylinder is unnecessary, and a single-acting mold clamping cylinder is provided.

A vertical single-acting mold clamping cylinder 20, a mold clamping ram 22 on one end of which a movable base for fitting a movable mold is fitted and which has an oil through-hole 35 communicated with an inner oil room from the outer peripheral face and is freely slidably inserted in the inner diameter face of the mold clamping cylinder, an overhead tank 24 with a purpose of prefill into the mold clamping cylinder 20 during mold clamping and fitted to the mold clamping cylinder 20, a prefill valve 28 for controlling opening/closing of an oil path between the mold clamping cylinder 20 and the overhead tank 24 and a boost cylinder 30 wherein an oil through-hole 29 which passes through in the axial direction is provided and one end is fixed on the opposite side of the mold fitting side of the mold clamping cylinder 20 and a piston head 31 at the apex part of another end is freely slidably inserted in the oil room of the mold clamping ram 22, are provided.