An operation abnormality detection method for detecting an operation abnormality in a molding device having a movable part includes: acquiring a prescribed measured value relating to operation of the movable part and calculating, as statistical information, an average value of prescribed information values based on the prescribed measured values: calculating a first threshold and a second threshold; acquiring a current measured value and comparing a current information value based on the current measured value with the greater of the first threshold and the second threshold; and issuing a warning if the current information value at least exceeds the greater of the first threshold and the second threshold.