To provide an injection quantity measuring device, easily measuring the injection quantity with good accuracy.

This injection quantity measuring device 1 measures the injection quantity of a fluid jetted from an injector 4. The device includes: a cylinder 5 disposed horizontally; a piston 8 slidably stored in the interior of the cylinder 5 and sealing and partitioning the interior into two chambers 6, 7; a fluid filling the interiors of both chambers 6, 7; a supply passage 10 connected to one chamber 6 to supply the fluid jetted from an injector 4 to the chamber 6 and move the piston 8 to the other chamber 7 side; a discharge passage 16 connected to the other chamber 7 to discharge the fluid filling the interior of the chamber 7 while keeping filling with the movement of the piston 8; meters 27, 28 disposed at least on one end side of the cylinder 5 to measure the sharing load; and an operating means for operating the travel of the piston 8 on the basis of variation of sharing load of the meters 27, 28 to obtain the injection quantity.