The inner cylindrical body is mounted on the top surface of the rudder-stock; the outer cylindrical body is put on the inner cylindrical body exteriorly; the gudgeon pin provided on the top reverse surface of the outer cylindrical body is inserted into the gudgeon provided on the top surface of the inner cylindrical body; the pin protruded on the top surface of the outer cylindrical body is inserted into the chest fixated to the supports; the rotational movement checking device, that obstructs the pin to rotate, and at the same time, permits it to be displaced in the radial direction, is provided inside the chest; the scale is provided on the outer circumferential surface of the inner cylindrical body; and, the rudder angle detecting optical sensors facing the scale are mounted on the inner circumferential surface of the outer cylindrical body.