Disclosed is a toroidal core current transformer comprising a primary conductor which penetrates the toroidal core and a secondary winding that is wound around the toroidal core. A measuring shunt (RSh) or a load (Zb) is provided at the secondary end. In order to compensate the phase error (f) between the primary current (IP) and the secondary current (Is), a compensation circuit (K) whose capacity (C) is measured substantially according to equation (I) or (II) is provided at the secondary end. In equation (I) or (II), C represents the capacity of the compensation circuit, ω represents the radian frequency, φ represents the phase error between the primary current and the secondary current, Rsh represents the resistance of an optional measuring shunt, and Zb represents the impedance of an optional load. C =  tan ⁢ ⁢ ϕ  ω · R sh ( I ) C =  tan ⁢ ⁢ φ  ω ·  Z _  ( II )