The apparatus supplies compressed air to molding sand (S) in a flask (26,27). A compressed air reservoir is constituted by a cover (14) and a surface-plate frame (7) having a compressed-air outlet port (6) which can be opened/closed by a vertically movable piston (12) and a sectioning devie (10) constituted by concentrically disposed large and small cylindrical members (10a,10b) secured to a cover member (10c) having air feed holes (11), between the cylindrical members (10a, 10b), and by connecting ducts (10d) for communicating the outside of the large cylindrical member (10a) with the inside of the small cylindrical member (10b). As an alternative to this, the sectioning device (60) is composed of a cylindrical member (60a) extending vertically and having a spur-like cross-sectional shape (Fig. 5) and a cover member (60b) hermetically secured to the lower end portion of the cylindrical member (60a) and having a spur-like air feed hole (61) communicated with the inside space of the cylindrical member (60a). When the piston (12) is moved upwards, compressed air in the reservoir is quickly discharged by a large quantity through the outlet port (6) via the sectioning device (10,60).