A mass spectrometric instrument, comprising: a circuit board on which electric elements are disposed, the electric elements including a capacitor forming a voltage-dividing circuit and a diode forming a detector circuit, and used in an electric circuit that feeds back a high-frequency voltage supplied to a mass filter of a mass spectrometer; a casing that houses the circuit board; a gas supply unit; and a mechanism that controls humidity and temperature of gas supplied from the gas supply unit and applies the gas with the humidity and temperature controlled to the capacitor and the diode, wherein the mechanism includes a piping unit that guides the gas with the humidity and temperature controlled to the circuit board, and a diaphragm for adjusting the gas is disposed at a connecting portion between the casing and the piping unit, the mass spectrometric instrument further comprising: a humidity/temperature measuring instrument that measures humidity and temperature of surrounding environment of the circuit board; temperature measuring units provided on the detector circuit and on the voltage-dividing circuit; a humidity controller that, on the basis of information on humidity from the humidity/temperature measuring instrument and information on temperature from the temperature measuring units on the detector circuit and the voltage-dividing circuit, calculates dew point temperature T1 of the surrounding environment of the circuit board, and calculates humidity that allows dew point temperature T2 of the gas to be lower than the dew point temperature T1; and a humidity regulator that regulates humidity of the gas supplied from the gas supply unit on the basis of the humidity calculated by the humidity controller. a circuit board on which electric elements are disposed, the electric elements including a capacitor forming a voltage-dividing circuit and a diode forming a detector circuit, and used in an electric circuit that feeds back a high-frequency voltage supplied to a mass filter of a mass spectrometer; a casing that houses the circuit board; a gas supply unit; and a mechanism that controls humidity and temperature of gas supplied from the gas supply unit and applies the gas with the humidity and temperature controlled to the capacitor and the diode, wherein the mechanism includes a piping unit that guides the gas with the humidity and temperature controlled to the circuit board, and a diaphragm for adjusting the gas is disposed at a connecting portion between the casing and the piping unit, the mass spectrometric instrument further comprising: a humidity/temperature measuring instrument that measures humidity and temperature of surrounding environment of the circuit board; temperature measuring units provided on the detector circuit and on the voltage-dividing circuit; a humidity controller that, on the basis of information on humidity from the humidity/temperature measuring instrument and information on temperature from the temperature measuring units on the detector circuit and the voltage-dividing circuit, calculates dew point temperature T1 of the surrounding environment of the circuit board, and calculates humidity that allows dew point temperature T2 of the gas to be lower than the dew point temperature T1; and a humidity regulator that regulates humidity of the gas supplied from the gas supply unit on the basis of the humidity calculated by the humidity controller. a humidity/temperature measuring instrument that measures humidity and temperature of surrounding environment of the circuit board; temperature measuring units provided on the detector circuit and on the voltage-dividing circuit; a humidity controller that, on the basis of information on humidity from the humidity/temperature measuring instrument and information on temperature from the temperature measuring units on the detector circuit and the voltage-dividing circuit, calculates dew point temperature T1 of the surrounding environment of the circuit board, and calculates humidity that allows dew point temperature T2 of the gas to be lower than the dew point temperature T1; and a humidity regulator that regulates humidity of the gas supplied from the gas supply unit on the basis of the humidity calculated by the humidity controller.