The invention relates to an infrared ray moisture meter which measures moisture of paper using absorption of infrared rays, and in which infrared rays being penetrated and scattered by the paper are sufficiently independent of whether the paper is thin or thick, in which sensitivity is high, in which attenuation of rays is small and error due to misalignment in the XY direction is small. Also, concurrently, in the invention, adverse influence due to quality of the paper is reduced by applying infrared rays which are absorbed by moisture, infrared rays which are absorbed by cellulose, and infrared rays which are not absorbed by moisture or cellulose, and by computing the value of moisture from signals detected from the application of the different infrared rays. In one embodiment a device is provided with a shielding plate having two mirrored surfaces disposed between the paper being measured and a lower one of a pair of reflectors.