A filter circuit for a biaxial signal, etc. which has an excellent sharp filtering characteristic in a narrow band, without performing conversion/inverse conversion from a coordinate system at rest to a rotatory coordinate system. Input signals related to two axes orthogonal to each other are expressed by one complex variable ( u = Ux + jUy: where Ux and Uy are Laplace transformation values of the respective signals). A complex coefficient transfer function F (s - j omega ) which is obtained by replacing the Laplacian s of F(s) is used as the transfer function of real number coefficient of a low- or high-pass filter. Output signals related to two axes orthogonal to each other and passing the input complex variable u through a complex coefficient transfer function F (s - j omega ) are expressed by one complex variable ( V = Vx + jVy: where Vx and Vy are Laplace transformation values of the respective signals). The connection is made with transfer elements of real number coefficient so that the real part and the imaginary part of a transfer expression equation which is obtained by multiplying both sides of a transfer expression equation U . F (s - j omega ) = V of U , F and V as defined above by the denominator of F (s - omega ) may be equal to each other.