A forming defect detection system for a three-dimensional additive manufacturing device which performs additive molding by emitting a laser beam to a powder bed is provided. This system specifies a candidate position of a forming defect on the basis of a change amount of a local temperature measured in an irradiated part of a powder bed irradiated by a laser beam. The system calculates a cooling speed at the candidate position on the basis of a temperature distribution and determines whether a forming defect exists on the basis of the cooling speed.