An experimental method is developed for determining the internal mass flux through frost with a dendrite structure. The theory and experimental procedures of the measurements using water slightly concentrated with deuterium are described. The internal mass flux determined from variations in the deuterium concentration in frost with time shows an almost linear decrease in the frost depth direction from the surface. The profile of the local frost density is almost flat in the depth direction. The frost uniformity is kept independent of the cooled-plate temperature and the total pressure in the container unless part of the frost melts. The cause for the uniformity of frost density is discussed in terms of the frost temperature profile and microscopic crystal growth.