A molten plastic extruded from extruding means 10 is formed into a strand 50 by a die 40, passed through an inner side of a cylindrical guide 60 erected vertically, thereafter, the strand 50 is successively cut uniformly to a constant length by a cutter 90 to form a pellet. A refrigerant for air cooling is circulated at a surrounding of the strand 50 passing through the inner side of the cylindrical guide 60 by cooling means 100 of the strand without being leaked to outside air.