A core sand filling method for a core molding machine using an under-blow type core sand filling device for filling a core box with core sand blown from thereunder; the core sand filling device comprising: the core box having a cavity to be filled with the core sand; a blow head having a sand blow chamber for blowing the core sand into the cavity and a sand storage chamber storing the core sand to be supplied to the sand blow chamber and communicating with the sand blow chamber; aeration air supply means for supplying the sand blow chamber with an aeration air for floating and fluidizing the core sand within the sand blow chamber; compressed air supply means for supplying the sand storage chamber with a compressed air; an exhaust valve for evacuating the compressed air remaining within the sand blow chamber; a first pressure sensor for measuring a pressure (Pf) within the sand blow chamber; a second pressure sensor for measuring a pressure (Pc) within the sand storage chamber; and a control unit for controlling respective operations of the aeration air supply means, compressed air supply means, and exhaust valve according to signals from the first and second pressure sensors; the core sand filling method comprising: a first step of supplying an aeration air into the sand blow chamber by the aeration air supply means in a state where the blow head and the core box communicate with each other while the blow head is located under the core box, so as to float and fluidize the core sand within the sand blow chamber; a second step of determining after the first step by the control unit whether the pressure (Pf) of the sand blow chamber measured by the first pressure sensor reaches a first pressure or not and, when it is determined by the control unit that the pressure (Pf) reaches the first pressure, supplying the compressed air into the sand storage chamber by the compressed air supply means according to an instruction from the control unit, so as to fill the cavity of the core box with the floated and fluidized core sand from within the sand blow chamber; a third step of determining after the second step by the control unit whether a first condition that each of the pressure (Pf) within the sand blow chamber and the pressure (Pc) within the sand storage chamber is at least a second pressure higher than the first pressure and a second condition that a differential pressure (ΔP = Pc - Pf) between the pressure (Pc) within the sand storage chamber and the pressure (Pf) within the sand blow chamber is a third pressure or lower are satisfied or not and, when both the first and second conditions are satisfied, stopping the respective operations of the aeration air supply means and compressed air supply means according to an instruction from the control unit; and a fourth step of instructing the exhaust valve to open by the control unit after the third step, so as to evacuate the compressed air from within the sand blow chamber. the core box having a cavity to be filled with the core sand; a blow head having a sand blow chamber for blowing the core sand into the cavity and a sand storage chamber storing the core sand to be supplied to the sand blow chamber and communicating with the sand blow chamber; aeration air supply means for supplying the sand blow chamber with an aeration air for floating and fluidizing the core sand within the sand blow chamber; compressed air supply means for supplying the sand storage chamber with a compressed air; an exhaust valve for evacuating the compressed air remaining within the sand blow chamber; a first pressure sensor for measuring a pressure (Pf) within the sand blow chamber; a second pressure sensor for measuring a pressure (Pc) within the sand storage chamber; and a control unit for controlling respective operations of the aeration air supply means, compressed air supply means, and exhaust valve according to signals from the first and second pressure sensors; a first step of supplying an aeration air into the sand blow chamber by the aeration air supply means in a state where the blow head and the core box communicate with each other while the blow head is located under the core box, so as to float and fluidize the core sand within the sand blow chamber; a second step of determining after the first step by the control unit whether the pressure (Pf) of the sand blow chamber measured by the first pressure sensor reaches a first pressure or not and, when it is determined by the control unit that the pressure (Pf) reaches the first pressure, supplying the compressed air into the sand storage chamber by the compressed air supply means according to an instruction from the control unit, so as to fill the cavity of the core box with the floated and fluidized core sand from within the sand blow chamber; a third step of determining after the second step by the control unit whether a first condition that each of the pressure (Pf) within the sand blow chamber and the pressure (Pc) within the sand storage chamber is at least a second pressure higher than the first pressure and a second condition that a differential pressure (ΔP = Pc - Pf) between the pressure (Pc) within the sand storage chamber and the pressure (Pf) within the sand blow chamber is a third pressure or lower are satisfied or not and, when both the first and second conditions are satisfied, stopping the respective operations of the aeration air supply means and compressed air supply means according to an instruction from the control unit; and a fourth step of instructing the exhaust valve to open by the control unit after the third step, so as to evacuate the compressed air from within the sand blow chamber.