A flask for containing mold sand within it prevents any mold shifting or mold dropping. Each of an upper flask 2 and a lower flask 3 includes a body that defines an opening in which a sand mold is to be molded. The body has at least one inlet 101 for introducing the mold sand into the opening. Two flanges 102 are extended from the body such that they are opposed to each other across the opening. Each flange has a through bore. The flask also includes engaging members for engaging an actuator in the outside of the flask such that a force or forces from said actuator could be transmitted to the flask. An upper flask 104 and a lower flask 105 are opposed to each other across a pattern plate 107. They are integrally assembled to make a flask unit by means of a pair of connecting rods 106 that are fitted in each bore.