In a cutter structure for a shield machine which advances by boring through a tunnel wall from inside an existing tunnel, the shape of an excavating surface on a work face formed by a cutter is set so as to have a smaller curvature than or an equal curvature to the curve of a surface to be excavated on the outer face of the tunnel wall to be excavated. In a first operating state a workload manager may be experiencing a first or a normal workload. In normal operation, the compression spring urges the bolt such that the shaft is at one end of the elongated opening. The actuators are fixed to the channel unit, each of which changes the volume of one of the pressure chambers.