PROBLEM TO BE SOLVED: To continuously measure change in the heat load of a specific object at a lapse of time under high heat load environment by substituting temperature being measured at a plurality of points where distance from the heat reception surface of an object to be measured is different into a specific expression.
SOLUTION: Temperature is measured at two points where distance from the heat reception surface of an object to be measured is set to Xa and Xb, and the measured temperatures Ta and Tb are substituted into an expression for calculating a heat load qin. In the expression, κ, Cp, and ρ are the thermal conductivity of the object to be measured, constant-voltage heat capacity, and the density of the object to be measured, respectively. For example, in a beam limiter being arranged near plasma so that a neutral particle beam transmitting the plasma enters a surface only, a circular through hole being extended in a direction where the neutral particle beam flows is formed. In the through hole, two measuring elements are arranged, and the temperature at two points in the direction where the neutral particle beam flows is measured. The output of the measuring elements is connected to a calorie calculation circuit for calculating change in the heat load of the neutral particle beam with time.