A blocking prevention device for a gasification melting system 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 including: a slag adhesion prevention device having a slag adhesion prevention capability for preventing adhesion of the slag at an opening part that may be blocked due to the adhesion of the slag; an imaging device that images the opening part; and a control device including a calculation unit that calculates a change rate of an opening area of the opening part using a plurality of images with different capturing times or a video, captured by the imaging device, and a prevention device control unit that changes the slag adhesion prevention capabilities of a plurality of the slag adhesion prevention devices in accordance with the change rate.