There are provided a carbon fiber reinforced prepreg of gas barrier properties comprising a sheet-like carbon fiber reinforcement and a matrix resin including therein a gas barrier layer having a clay mineral with a plate-like crystal structure unidirectionally aligned and densely laminated, and a carbon fiber reinforced plastic produced from the carbon fiber reinforced prepreg. One or more equalizers are each provided on one of the paths and are operative to equalize signal amplitude. The output signals are processed by a controller or computer to provide a measurement of the moisture stratification.