An interference checking device 1 checks, during machining using an NC machine tool 20, interference between structures of the NC machine tool 20, a tool and a workpiece, and includes an interference check processing unit 2 virtually moving models and checking the occurrence of interference based on a position control signal from a numerical controller 30, and an operation effect degree evaluating unit 6 evaluating the degree of an effect of an operation on a possibility of the occurrence of interference. When a predetermined operation is performed during machining of a workpiece following a first workpiece, the interference check processing unit 2 moves the models at intervals determined based on the degree of the effect evaluated by the operation effect degree evaluating unit 6.