The present invention makes it possible to suppress variations in the weight of a high viscosity substance at each extrusion of the high viscosity substance and each cut-off to the extent possible. Metallic sodium (5) filled into an extruder main body (3) is extruded from a nozzle (7) of the extruder main body (3) by a pressing tool (4), and when extrusion of the metallic sodium (5) to a prescribed position is detected, movement of the pressing tool (4) is stopped and then the extruded metallic sodium (5) is cut. In this instance, the impulse (P) after stopping until the load ceases to act on the metallic sodium (5) within the extruder main body (3) after the movement of the pressing tool (4) has stopped is made to approach a constant value by adjusting the speed of movement of the pressing tool (4).
