PURPOSE: To obtain the amplitude value of the signal from the values of two samples in a linear form, by providing a constant memory part, an operating means, and the like, by obtaining the addresses of said constant memory part from the output of a sampling means, and performing the operation by using the constant stored in said addresses.

CONSTITUTION: An AC signal Q such as a current, voltage, or the like is applied to the sampling measns 1 via a current transformer CT and the like from an electric power system. Said means 1 samples and holds the input AC signal Q at a specified time interval. The output of said means 1 is converted into a digital signal by an A/D converter 2 and outputted. The operating means 3 receives the digital signal D, executes the specified operation, and supplied an address signal A to the constant memory 4. Then, the operating means 3 receives a constant C from the constant memory part 4 based on the singal A and executes a specified operation using the constant C and the digital signal D, and sends out the output P. In this constituion, the amplitude value of the AC signal is obtained from the two sampled values which are not limited to a specifiec value in a linear form.