NOVELTY - A gas composition estimation program involves numerical assumption step which assumes ratio of amount of substance of gaseous phase in sealed space and preparation amount of substance and ratio of molar fraction of each component of gaseous phase in sealed space and molar fraction of each component of liquid phase to arbitrary numerical values, and output step which outputs molar fraction of each component computed at molar fraction calculation step using computer when all conditions are satisfied in condition determination step.    USE - Gas composition estimation program for gas combustor (e.g. engines for motor vehicle, boiler, water heater, gas cooking stove and gas stove). Can also be used for manufacturing fuel gas, town gas, gas for gas lighters, motor gas (fuel for taxis) and gas for fuel cells (hydrogen source of fuel cell) for household field and business field.    ADVANTAGE - The program quantitatively and easily estimates composition of vaporization gas vaporized from multi-component liquefied gas of arbitrary composition.    DETAILED DESCRIPTION - A gas composition estimation program involves input step which waits for input of molar fraction (z) of each component, temperature (T), volume (V) of sealed space, preparation amount of substance (F) of liquefied gas and prepared liquefied gas, numerical assumption step which assumes ratio (D/F) of amount of substance (D) of gaseous phase in sealed space and preparation amount of substance (F) and ratio (y/x) of molar fraction (y) of each component of gaseous phase in sealed space and molar fraction (x) of each component of liquid phase to arbitrary numerical values while assuming pressure (P) in sealed space based on molar fraction (z) of each component, temperature (T) inputted at input step, volume (V) of sealed space and preparation amount of substance (F), molar fraction calculation step which computes molar fraction (y) of each component while computing molar fraction (x) of each component, molarity calculation step which computes molarity (( rho )L) of liquid phase in sealed space and molarity (( rho )V) of gaseous phase using Benedict-Webb-Rubin characteristic equation, fugacity calculation step which computes fugacity of each component, distribution rate calculation step, condition determination step which determines whether conditions are satisfied, repetition step which repeats operation of numerical assumption step, molar fraction calculation step, molarity calculation step, fugacity calculation step and distribution rate calculation step until all conditions are satisfied, and output step which outputs molar fraction (x) of each component computed at molar fraction calculation step using computer when all conditions are satisfied in condition determination step. An INDEPENDENT CLAIM is included for a gas composition estimation apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of the calculation method of three-component-type vapor-liquid equilibrium relationship at the time of considering volume income and expenditure. (Drawing includes non-English language text).