Electromagnetic vibrations are induced in metallic materials during solidification by simultaneous application of alternating electric and stationary magnetic fields. Cavities form and collapse on the surface of solid crystals by application of electromagnetic vibration and then a huge pressure is exerted on the surroundings when they collapse. This pressure may result into the refinement of solid crystals. This mechanism has been applied to aluminum alloys, magnesium alloys and cast irons during solidification. It has been clarified that primary solid crystals are extensively refined.