A gas supply system comprising: a fuel tank to which a gas is supplied from a gas tank of a gas station; a gas flowmeter that measures an amount of the gas supplied from the gas tank to the fuel tank; a temperature sensor that measures a temperature of the gas in the fuel tank; a pressure sensor that measures a pressure of the gas in the fuel tank and generates an output signal in accordance with the pressure; and a control portion that corrects a relationship between the pressure applied to the pressure sensor and a value of the output signal, wherein the control portion acquires a first temperature of the gas in the fuel tank, a first pressure of the gas in the fuel tank, and a first output signal value of the pressure sensor corresponding to the first pressure before supply of the gas, the control portion acquires a first amount of the gas in the fuel tank using a volume of the fuel tank, the first temperature, and the first pressure, the control portion acquires a second amount of the gas in the fuel tank after supply of the gas, through integration of a measurement value obtained by the gas flowmeter during supply of the gas, the control portion acquires a second temperature of the gas in the fuel tank, a second pressure of the gas in the fuel tank, and a second output signal value of the pressure sensor corresponding to the second pressure after supply of the gas, the control portion calculates a third pressure of the gas in the fuel tank using the volume of the fuel tank, the second amount of the gas, and the second temperature, and the control portion corrects the relationship between the output signal of the pressure sensor and the pressure in the fuel tank using the first pressure, the first output signal value, the third pressure, and the second output signal value. a fuel tank to which a gas is supplied from a gas tank of a gas station; a gas flowmeter that measures an amount of the gas supplied from the gas tank to the fuel tank; a temperature sensor that measures a temperature of the gas in the fuel tank; a pressure sensor that measures a pressure of the gas in the fuel tank and generates an output signal in accordance with the pressure; and a control portion that corrects a relationship between the pressure applied to the pressure sensor and a value of the output signal, wherein the control portion acquires a first temperature of the gas in the fuel tank, a first pressure of the gas in the fuel tank, and a first output signal value of the pressure sensor corresponding to the first pressure before supply of the gas, the control portion acquires a first amount of the gas in the fuel tank using a volume of the fuel tank, the first temperature, and the first pressure, the control portion acquires a second amount of the gas in the fuel tank after supply of the gas, through integration of a measurement value obtained by the gas flowmeter during supply of the gas, the control portion acquires a second temperature of the gas in the fuel tank, a second pressure of the gas in the fuel tank, and a second output signal value of the pressure sensor corresponding to the second pressure after supply of the gas, the control portion calculates a third pressure of the gas in the fuel tank using the volume of the fuel tank, the second amount of the gas, and the second temperature, and the control portion corrects the relationship between the output signal of the pressure sensor and the pressure in the fuel tank using the first pressure, the first output signal value, the third pressure, and the second output signal value.