PURPOSE: To detect surely from an outside a residue quantity of a gas in a cassette type gas cylinder accommodating a liquefied gas.

CONSTITUTION: A fact that a resonance frequency does not change by a residue quantity of a gas in a cassette type gas cylinder 1, is noticed, and the cylinder 1 is set in a gas appliance. A transmitter device 11 and a receiver 12 are set in a position wherein an output electric voltage of the transmitter 12 becomes equal to or greater than a specific value at respectively the specified resonance frequency and a gas residue quantity which is intended to detect, with a position opposing to a welded section of a cylinder shell 2 of the gas cylinder as a reference. And, the gas residue quantity of a liquefied gas in the cylinder is detected by detecting a signal reception output value equal to or greater than the specific value.