Casting defects caused by the core gas evolution impair the performance of a machine. The estimation of core gas evolution and core out-gassing are necessary for thin-wall high-pressure castings with complicated actual size core systems. The development of our patented device for core out-gassing made possible a systematical approach for the vent design of core out-gassing in the design stage of the core shape and the mold concepts of casting, such as engine parts with complicated core systems. Experimental procedures and analytical reviews on the relative performance evaluation of suction routes of core gas were attempted with aluminum thin-wall cylinder head casting. Also, two different distributions of particle sizes of core sand were evaluated in order to review effects of fine particle of core sand on core out-gassing. The device developed for core out-gassing in this study was composed of a chamber, a vacuum pump, pressure sensors, and a data logger.