β-FeSi2 is expected to produce an efficient bandpass filter for its bulk is transparent and has a high refractive index (for wavelength＞1.55 µm). However, it was found that β-FeSi2 thin film has a different extinction coefficient from the bulk and a significant absorptance in the infrared region. We carried out experiments to research the relationship between the absorptance and composition of β-FeSi2 thin film using co-sputtering by control the applied power on FeSi2 and Si targets. The β-FeSi2 thin film deposited at an applied power of 10～12 W on Si target had a correct stoichiometry and the minimum absorptance of approximately 0. It is suggested that high refractive index and low extinction coefficient in the infrared spectrum of β-FeSi2 thin film can be successfully produced in this method and used for bandpass filters to enhance the efficiency in the Thermophotovoltaic power generation system.