Injection rate of a gaseous fuel injector was calculated by measuring the needle valve lift history. In our lab, a gaseous fuel injection rate meter using 1-dimensional pipe flow was developed. In the present study, the measurement accuracy of the injection rate meter was examined by comparing the theoretical injection rate estimated from the needle valve lift history. The needle valve lift was measured using an optical method. A small beam cut plate is attached to the needle valve of the gaseous fuel injector. A laser beam passes through the beam cut plate, and the variation of the beam intensity was measured with a photo sensor. From the beam intensity, the needle valve lift history was estimated. One-dimensional, compressible, inviscid, adiabatic flow was assumed, and the instantaneous mass flow rate through the needle valve sheet, the injector sack, and the injection hole was estimated. The estimated injection rate was compared with that obtained with the gaseous fuel injection rate meter. The estimated and measured injection rates agree relatively well even at transient duration. It is shown that the injection rate meter has high reliability and accuracy.