An apparatus for forming a porous preform that is useable to an optical fiber by extruding a plastifiable material having silica powder as a main component thereof. The apparatus includes an extruding main body having a cylinder and a screw for extruding the plastifiable material from the cylinder and a material passage member connected to the cylinder so as to communicate therewith. The material passage member has a projection member which projects into the material passage member for preventing a lamination of the plastifiable material. The apparatus also includes a molding head connected to the material passage member so as to communicate therewith.