A blocking prevention device (11, 12) for a gasification melting system that combusts and melts an object to be treated into a slag in a melting furnace after the object to be treated is converted into pyrolysis gas in a gasification furnace, the blocking prevention device comprising: a plurality of slag adhesion prevention devices that each have a slag adhesion prevention capability for preventing adhesion of the slag at an opening part (16, 20) that may be blocked due to the adhesion of the slag; an imaging device (39, 40) configured to image the opening part (16, 20); and a control device including a calculation unit configured to calculate a change rate of an opening area of the opening part (16, 20), using a plurality of images with different capturing times or a video, captured by the imaging device (39, 40), and a prevention device control unit configured to change slag adhesion prevention capabilities of a plurality of the slag adhesion prevention devices in accordance with the change rate, the blocking prevention device being characterized in that: the prevention device control unit is configured to change the slag adhesion prevention capabilities of the plurality of slag adhesion prevention devices on the basis of the opening area and the change rate, in that the prevention device control unit is configured to operate: all the slag adhesion prevention devices with a first slag adhesion prevention capability higher than a predetermined slag adhesion prevention capability in a case where the change rate is larger than a first change rate (bs, bo) and the opening area is smaller than a first opening area (Bs, Bo), and all the slag adhesion prevention devices with a second slag adhesion prevention capability lower than the predetermined slag adhesion prevention capability in a case where the change rate is larger than the first change rate (bs, bo) and the opening area is larger than a second opening area (As, Ao) which is larger than the first opening area (Bs, Bo). a plurality of slag adhesion prevention devices that each have a slag adhesion prevention capability for preventing adhesion of the slag at an opening part (16, 20) that may be blocked due to the adhesion of the slag; an imaging device (39, 40) configured to image the opening part (16, 20); and a control device including a calculation unit configured to calculate a change rate of an opening area of the opening part (16, 20), using a plurality of images with different capturing times or a video, captured by the imaging device (39, 40), and a prevention device control unit configured to change slag adhesion prevention capabilities of a plurality of the slag adhesion prevention devices in accordance with the change rate, the blocking prevention device being characterized in that: the prevention device control unit is configured to change the slag adhesion prevention capabilities of the plurality of slag adhesion prevention devices on the basis of the opening area and the change rate, in that the prevention device control unit is configured to operate: all the slag adhesion prevention devices with a first slag adhesion prevention capability higher than a predetermined slag adhesion prevention capability in a case where the change rate is larger than a first change rate (bs, bo) and the opening area is smaller than a first opening area (Bs, Bo), and all the slag adhesion prevention devices with a second slag adhesion prevention capability lower than the predetermined slag adhesion prevention capability in a case where the change rate is larger than the first change rate (bs, bo) and the opening area is larger than a second opening area (As, Ao) which is larger than the first opening area (Bs, Bo). all the slag adhesion prevention devices with a first slag adhesion prevention capability higher than a predetermined slag adhesion prevention capability in a case where the change rate is larger than a first change rate (bs, bo) and the opening area is smaller than a first opening area (Bs, Bo), and all the slag adhesion prevention devices with a second slag adhesion prevention capability lower than the predetermined slag adhesion prevention capability in a case where the change rate is larger than the first change rate (bs, bo) and the opening area is larger than a second opening area (As, Ao) which is larger than the first opening area (Bs, Bo).