PROBLEM TO BE SOLVED: To improve productivity by increasing the number of cores manufactured per one cycle in a horizontal split type core manufacturing machine. SOLUTION: A core box structure 23 is provided with a lower side core box 32 and an upper core box 40 over it. The lower side core box 32 is composed of a lower side cope 33 and a lower side drag 35 under it, and a lower side cavity 38 is formed with both components. The upper core box 40 is composed of an upper side cope 41 and an upper drag 44 under it, and an upper side cavity 47 is formed with both components. A passage 50 penetrating the upper core box 40 and extending to the lower core box 32 is provided, and sands being a core material are fed to both cavities 38 and 47 through the passage 50. After cores are hardened, the core box structure 23 is opened and cores are taken out.