To continuously measure the layer thickness and layer weight of a sheet like matter in-line with non-destruction and non-contact without stopping the production by converting them from an infrared ray transmission measured value of a specified wavelength in which the transmittance ratio in a specified infrared wavelength area shows absorption a specified magnification or less of the transmittance in a flat part.

A layer thickness and layer weight of a layer to be measured are measured in-line by converting them from an infrared ray transmission measured value in a specified wavelength where the transmittance ratio of an infrared ray having a wavelength of 1.5-5 μm is 0.8 times or less, compared with that in a flat part, and the transmittance of the other layer is flat in this wavelength. An infrared ray source 1, a spectral element 2, and a light receiving part 3 are arranged on a frame 4 at an equal pitch interval P. The frame 4 is reciprocated at the pitch P or more, and a sheet like matter 5 which is a matter to be measured is traveled vertically thereto. Standard calibration matters 61, 52 are provided on the lateral sides of the sheet 5 to impart a double redundancy by the calibration according to the arrowed order of waveform by these, so that the reliability of calibration can be improved.