A method for diagnosing a defect in a rotating machine, according to the present disclosure, may comprise the steps of: determining a defect level on the basis of data obtained by diagnosing the state of the rotating machine, the data, obtained by diagnosing the state of the rotating machine, including at least one from among a feature vector related to a vibration signal of the rotating machine, a frequency linked to the defect in the rotating machine and the total vibration value of the rotating machine; applying a weight to the defect level on the basis of information related to a defect in state history data of the rotating machine and/or whether an alarm related to operating information about the rotating machine has occurred; and determining the defect severity of the rotating machine on the basis of the defect level to which the weight is applied.