PURPOSE: To shorten the time for measurement by determining the max. values and min. values within a prescribed unit period for each of unit periods successively and obtaining the average value of each of a prescribed number of the unit periods.

CONSTITUTION: A semiconductor integrated device 1 to be measured is operated in a test pattern STP and the current is detected in a power source current detecting part 2 and is converted to a voltage. Max. value and min. value holding circuits 3a1W3an, 3b1W3bn are successively switched in each unit period TU by switching circuits 4a, 4b and detect, hold and output the max. values and min. values of the output voltage within the period TU. The max. values and min. values are added for each period TU by adders 6a1W6an, 6b1W6bn and the average values of each period TU are determined by dividers 7a1W7an, 7b1W7bn. The average values are added by adders 8a, 8b and are divided by 1/n by dividers 9a, 9b to determine the average value within the period where n-pieces of the periods TU are continuously aggregated. The output signals of average computing parts 5a, 5b are alternately switched by each of n-pieces of the periods TU and are outputted. The outputted signals are then converted by a converter 11 and are transmitted to a CPU 12. The time for measuring the average power source current is thus shortened.