a plastic case and a method for producing a plastic case composed of two halves ultrasonically welded together without problems caused by the generation of excess material , namely , excess melted resin and resin particles , produced during the welding process . the welding is effected at abutting portions of a projection formed on one of the two case halves and a recess formed on the other . a gap is left between the end surface of the distalface of the projection and the bottom of the recess defining an internal space . at least one air hole is formed communicating with the internal space , and suction may be applied to the internal space through the air hole . also , a dust - blocking rib may be provided on at least one of the recess and projection surrounding the abutting welding portions .