PURPOSE: To accurately measure the transmittance of a reservoir lock in a short time by directly measuring the time width and the change of gas pressure when gas flows from the reservoir having accurately known volume to core sample.

CONSTITUTION: After a core sample 16 is suitably disposed in the channel 191 of a core rack 14, the depth of reference is input to a computer at each sample 16 to be measured in the rack 14. Then, a probe 22 is, for example, disposed until it is arrived at the position directly above the front edge of the reference depth line on the sample 16 to be measured. Thereafter, gas such as, for example, nitrogen is supplied from a pressurized gas source to a reservoir system and the probe 22 via a charging valve. When the gas flows in the sample 16 via the probe 22, the pressure of the gas is measured at a selected interval, the instantaneous pressureattenuation parameter are calculated based on the measurement of the pressure-time, and the transmittance of the sample 16 is decided.