A methane number calculation method comprising: acquiring in advance a particular relational expression between a methane number and a basic calorific value of a plurality of types of reference gases, each formed of a natural gas and each having a different value of the methane number; measuring a refractive index and a sonic speed of a natural gas, which is a sample gas to be measured, and measuring a concentration of a carbon dioxide gas being contained in the sample gas; calculating a refractive index converted calorific value Q o based on the measured refractive index from the correlation between the refractive index and the calorific value of a particular gas formed of only combustible components; calculating a sonic speed converted calorific value Q s based on the measured sonic speed from the correlation between the sonic speed and the calorific value of the particular gas formed of only combustible components; calculating a concentration of a nitrogen gas X N2 contained in the sample gas by the following formula, on the basis of the refractive index converted calorific value Q o, the sonic speed converted calorific value Q s and the concentration of the carbon dioxide gas X CO2: [Mathematical Expression 1] X N 2 = 100 k N 2 0 0 − 0 S 1 − α − k CO 2 X CO 2 100 where α is an correction factor and is a value selected within a range of 1.1 to 4.2, k N2 is an error coefficient of a nitrogen gas and k CO2 is an error coefficient for a carbon dioxide gas; calculating a calorific value Q os of the sample gas by the following formula, on the basis of the refractive index converted calorific value Q o and the sonic speed converted calorific value Q s: [Mathematical Expression 2] Q 0 S = Q 0 − Q 0 − Q S 1 − α Q 0 ≦ Q S where α is an correction factor and is a value selected within a range of 1.1 to 4.2; and calculating a methane number of the sample gas from a value of the basic calorific value Q', the value of the concentration of the nitrogen gas and the value of the concentration of the carbon dioxide gas, and the particular relational expression, wherein the basic calorific value Q' refers to the combustion calorific value of the combustible gas component acquired by removing the inert gas components from the natural gas and is calculated by the following formula, on a basis of the calorific value Q os, the concentration of the carbon dioxide gas X CO2 and the concentration of the nitrogen gas X N2: [Mathematical Expression 3] Q ′ = Q 0 S 1 − 0.01 X N 2 + 0.01 X CO 2. acquiring in advance a particular relational expression between a methane number and a basic calorific value of a plurality of types of reference gases, each formed of a natural gas and each having a different value of the methane number; measuring a refractive index and a sonic speed of a natural gas, which is a sample gas to be measured, and measuring a concentration of a carbon dioxide gas being contained in the sample gas; calculating a refractive index converted calorific value Q o based on the measured refractive index from the correlation between the refractive index and the calorific value of a particular gas formed of only combustible components; calculating a sonic speed converted calorific value Q s based on the measured sonic speed from the correlation between the sonic speed and the calorific value of the particular gas formed of only combustible components; calculating a concentration of a nitrogen gas X N2 contained in the sample gas by the following formula, on the basis of the refractive index converted calorific value Q o, the sonic speed converted calorific value Q s and the concentration of the carbon dioxide gas X CO2: [Mathematical Expression 1] X N 2 = 100 k N 2 0 0 − 0 S 1 − α − k CO 2 X CO 2 100 where α is an correction factor and is a value selected within a range of 1.1 to 4.2, k N2 is an error coefficient of a nitrogen gas and k CO2 is an error coefficient for a carbon dioxide gas; calculating a calorific value Q os of the sample gas by the following formula, on the basis of the refractive index converted calorific value Q o and the sonic speed converted calorific value Q s: [Mathematical Expression 2] Q 0 S = Q 0 − Q 0 − Q S 1 − α Q 0 ≦ Q S where α is an correction factor and is a value selected within a range of 1.1 to 4.2; and calculating a methane number of the sample gas from a value of the basic calorific value Q', the value of the concentration of the nitrogen gas and the value of the concentration of the carbon dioxide gas, and the particular relational expression, wherein the basic calorific value Q' refers to the combustion calorific value of the combustible gas component acquired by removing the inert gas components from the natural gas and is calculated by the following formula, on a basis of the calorific value Q os, the concentration of the carbon dioxide gas X CO2 and the concentration of the nitrogen gas X N2: [Mathematical Expression 3] Q ′ = Q 0 S 1 − 0.01 X N 2 + 0.01 X CO 2.