To operate and adjust an output to be fed to a dryer in operation of running-on by a precise way capable of being verified, by a method wherein the output of the dryer is varied according to a state quantity indicating a scale of a dryness degree of printed matter from a measured intensity of a microwave ray.

A microwave measuring apparatus 8 arranged between a varnishing unit 6 of a printing machine 4 and its dryer 2, has a microwave originating device radiating a microwave ray of which the wavelength is principally 0.5 mm to 4 cm toward a surface of a printed varnished sheet 10. The microwave ray executes mutual operation with a varnished film on the sheet 10 and is partially absorbed. A wavelength or a frequency of the microwave ray is selected so that varnish of the varnished film absorbs more the microwave ray in comparison with a material of the matter to be printed. A microwave originating device has a radiation output of not more than 150 mW.