PURPOSE: To obtain ground fish meat free of sensed nonmetallic rubber, glass, etc., by arranging a transmitter and receivers for ultrasonic waves in a nozzle part for extruding the ground fish meat under pressure and sensing foreign materials from a change in wave form of transverse waves.

CONSTITUTION: Fish meat free of metallic foreign materials is milled to provide ground fish meat and extruded from an extrusion outlet 23 made of a steel material. An ultrasonic transmitter 24 is arranged at the top surface of the extrusion outlet 23 and plural transverse wave receivers 25 are placed on the undersurface to receive ultrasonic waves propagated in the ground fish meat 40. The received ultrasonic waves are then compared with the normal wave form in a memory 34 with a wave form analyzer 35 to sense the foreign materials 41, which are subsequently removed with a foreign material removing controller 36 to afford the objective ground fish meat.