in order to prevent casting with an abnormal hollow core having partially thin wall or filled in parts , a new method suitable for forming good hollow core stably and suitable for checking the hollowness of the moulded core before casting is proposed . this method comprises a step of providing a filling hole and an air inlet hole in a mould for core formation , a step of filling the mould with core sand with the air inlet hole in a closed state , a step of heating the mould and a step of sucking out unhardened sand through the filling hole with the air inlet hole in an open state . unhardened sand is sucked out with the air flow from the air inlet hole to the filling hole , and the hollowness of the core is checked by measuring the pressure at the filling hole .