A method of determining a decrease of an optical output power in an optical fiber laser apparatus operable to output high-power light when pumped by pumping light from a pumping light source, the method comprising the steps of: acquiring a reference current I r supplied to the pumping light source when a predetermined reference optical output power P r that is equal to or greater than a maximum optical output power being used during an initial state of operation is outputted from the optical fiber laser apparatus; detecting a current I i supplied to the pumping light source during an operation of the optical fiber laser apparatus; time-averaging the detected current I i with a predetermined time constant to calculate a time-averaged current I av; detecting an optical output power P i outputted from the optical fiber laser apparatus during the operation of the optical fiber laser apparatus; time-averaging the detected optical output power P i with the predetermined time constant to calculate a time-averaged optical output power P av; calculating an optical output power expectation value P ex represented by P ex = I av × P r / I r from the time-averaged current I av, the reference optical output power P r, and the reference current I r; comparing the time-averaged optical output power P av with the optical output power expectation value P ex to calculate a difference (P ex - P av) between the optical output power expectation value P ex and the time-averaged optical output power P av or a ratio (P av / P ex) of the time-averaged optical output power P av to the optical output power expectation value P ex; and determining that the optical output power of the optical fiber laser apparatus is lowered when the difference is greater than a predetermined difference threshold or when the ratio is smaller than a predetermined ratio threshold. acquiring a reference current I r supplied to the pumping light source when a predetermined reference optical output power P r that is equal to or greater than a maximum optical output power being used during an initial state of operation is outputted from the optical fiber laser apparatus; detecting a current I i supplied to the pumping light source during an operation of the optical fiber laser apparatus; time-averaging the detected current I i with a predetermined time constant to calculate a time-averaged current I av; detecting an optical output power P i outputted from the optical fiber laser apparatus during the operation of the optical fiber laser apparatus; time-averaging the detected optical output power P i with the predetermined time constant to calculate a time-averaged optical output power P av; calculating an optical output power expectation value P ex represented by P ex = I av × P r / I r from the time-averaged current I av, the reference optical output power P r, and the reference current I r; comparing the time-averaged optical output power P av with the optical output power expectation value P ex to calculate a difference (P ex - P av) between the optical output power expectation value P ex and the time-averaged optical output power P av or a ratio (P av / P ex) of the time-averaged optical output power P av to the optical output power expectation value P ex; and determining that the optical output power of the optical fiber laser apparatus is lowered when the difference is greater than a predetermined difference threshold or when the ratio is smaller than a predetermined ratio threshold.