PURPOSE: To eliminate dead zone during an interval from the start of measurement to an average operating time by measuring the plate thickness based on the weighted average of data for every reference counting time during that interval.

CONSTITUTION: Radiation from a radiation source 2 transmits through an object to be measured (thick plate) 1 in the arrow direction and impinges on a detector 3. A detected signal 3 is processed at an input circuit section 5 which delivers a pulse signal corresponding to the plate thickness to an processing unit 6. The processing unit 6 counts the number of pulses every reference counting time and for the interval from start of measurement to an average operating time, a weighted average AXn of data is determined for every set reference counting time according to a formula I {in the formula, k: average operating time/reference counting time (fractions are raised to a unit), n: number of counts x}. Upon reaching the average operating time, moving average BXn of the counts (x) for every reference counting time is determined according to formula II. This method eliminates the dead zone and realizes a highly accurate high response thickness gauge.