A mold device and a method of manufacturing a cylinder block. The mold device comprises a split core (16) having two first split cores (46) formed so that the tip parts thereof may be in a tapering shape, two second split cores (50) installed between the first split cores (46), and an inner core (42) is installed at the center part and pushing out the first split cores (46) in the direction receding from the center axis thereof. When the inner core (42) is pushed out to push out and position the first split cores (46), both end parts of the second split cores (50) are brought into contact with the tip parts of the adjacent first split cores (46), and the outer side-faces (46a) of the first split cores (46) and the outer side-faces (50a) of the second split cores (50) form a cylindrical shape. When the inner core (42) is raised, the first split cores (46) and the second split cores (50) are pulled to the axial center (C) by first engagement pieces (67) and second engagement pieces (66).