Abstract: A high frequency filter for an electric instrument including a conductive casing insulated on a stationary structure, an internal electric circuit arranged within the casing, and a connector mounted on the casing for connecting the internal electric circuit to an external electric circuit. The filter comprises an insulation thin plate arranged between the connector and the internal electric circuit and being folded at its intermediate portion, a pair of conductive thin plates secured to opposite surfaces of the insulation thin plate and extending along the folded portion of the insulation thin plate, one of the conductive thin plates being connected at its one end to the connector and at its other end to the internal electric circuit, and the other conductive thin plate being connected to a portion of the casing to provide a short circuit. In such arrangement, the conductive thin plates are opposed to each other through the insulation thin plate to provide a distributed constant circuit.