Abstract: A liquefied gas monitoring system includes: a container containing liquefied gas, which is obtained by liquefaction of single-component gas; a liquid level sensor, which detects liquid level height in the container; and control circuitry, which receives, each time the liquefied gas is fed into the container, an input of an environmental value of the gas. Before the liquefied gas is fed into the container, the control circuitry determines an amount of the liquefied gas previously fed into and still remaining in the container based on the height detected by the liquid level sensor, and stores the determined amount together with the environmental value. After the liquefied gas is fed into the container, the control circuitry determines an amount of the liquefied gas fed at this time based on the height detected by the liquid level sensor, and stores the determined amount of the liquefied gas together with the environmental value.