At a heat treatment temperature in a reducing atmosphere of Ar and H.sub.2, a precursory film of V.sub.2 O.sub.5 is reduced into a VO.sub.x film with the heat treatment temperature selected in a predetermined temperature range between 350 .degree. C. and 450.degree. C., both exclusive, to control a resistivity of the VO.sub.x film, where x is greater than 1.875 and less than 2.0. The VO.sub.x film is not susceptible to a metal-semiconductor phase transition inevitable in VO.sub.2 at about 70.degree. C. and is excellent for use in a bolometer-type infrared sensor. When reduced at 350.degree. C. and 450.degree. C. the resistivity and its temperature coefficient of the VO.sub.x film at room temperature are 0.5 and 0.002 .OMEGA. cm and -2.2% and 0.2% per degree Celsius.