To enable measurement of a voltage to ground or a zero-phase voltage which is accurate for a long term, by obtaining column vector of n-rows by making a specific real number diagonal matrix of n-rows and n-columns act upon column vector of n-rows which is obtained by making a specified real number diagonal matrix of n-rows and n-columns act upon column vector the of n-rows of a potential difference measured value of a conductor to be measured.

The voltage of each phase which is transmitted with an optical signal is subjected to O/E conversion 6. An operating equipment 11 outputs a 3-row column vector [G] [v] by inputting the following; column vector of 3-rows [v] wherein the output v1 of an O/E conversion part 6 of a distribution line of a first (1≤3; integer) phase is the element of a first column, and a specified real number diagonal matrix of 3-columns and 3-rows [G] which is stored in a storage device 14. An operating equipment 12 outputs column vector of 3-rows [R] [G] [v] by inputting the output [G] [v] of the operating equipment 11 and a specified real number matrix [R] of 3-rows and 3-columns. An operating equipment 13 outputs the total sum V0 of each row of the column vector of 3-rows [R] [G] [v] by inputting the output [R] [G] [v] of the operating equipment 12.