PURPOSE: To perform the measurement easily in a high speed by obtaining a difference of a forward voltage obtained by impressing continuous pulses in the forward direction of the P-N junction of a sample and using it as a thermal resistance measuring means of the sample.

CONSTITUTION: A pulse power 12 is impressed to a diode laser 11 as a sample to be measured. Then, a current is flowed in the direction of an arrow, and a forward current value indicated by a bar graph is obtained. A forward current value (I) is taken as the axis of ordinates and a time (T) is taken as the axis of abscissas, and a multistage continuous pulse generator which increases the impressed voltage of the pulse power 12 stepwise with time is used to obtain forward current values 13 shown in a figure. The forward current is raised according as the elapse of time, and the correlation between a pulse voltage V and the forward current I at each time is obtained to obtain static characteristic curves 14 and 15. These two curves 14 and 15 are examples indicating characteristics of two kinds of diode laser 11, and the static characteristic curve 14 indicating a rapid rise indicates an abnormal value, and the diode laser element having this curve 14 includes some defect. The other static characteristic curve 15 indicates a normal element characteristic, and it is possible to discriminate whether the diode laser is good or not by this comparison.