A high power filter apparatus which is used in a mobile communication base station or the like is provided wherein the temperature stability and frequency selection are excellent, an insertion loss is small, the size is small, power consumption is low and costs are low. A shield case block comprises signal input and output portions, and a plurality of closed spaces which house a filter element connected between the signal input and output portions. A cooling plate is provide in a heat insulating container which houses the shield case block. The shield case block is fixed to the cooling plate in the thermal contact state. Each filter element is place almost in parallel. A cylindrical hole having an axis which is almost parallel with the face of the filter element penetrates the shield case block. A ground rod made of a conductor which changes the volume of the closed space is provided on the inner end portion of a movable member which moves in the axial direction of the cylindrical hole.