PURPOSE: To obtain a versatile maximum demand wattmeter with high reliability by storing a program in a memory, and reading and processing logically the pulse signal of the watt-hour meter of a circuit to be measured and displaying the processing result.

CONSTITUTION: Pulses are sent out of the watt-hour meter 1, and a counter 13 holds some value. When the command for the start of measurement is supplied at this time, MPU (microprocessor) 10 stores the value of the counter 13 in the memory 11 through an interface 12. New data is read out of the counter 13 through the interface 12 again at a set time in the program. This data and previously fetched data are processed by logical subtraction. Further, the relation between the number of pulses and an electric power value is specified in the program, so a maximum electric power value is calculated and stored in the memo- ry 11 to display the maximum electric power value on a display device 14.