A damage evaluation method, which evaluates a damage degree of a target portion which changes over time in a structural member, comprising: a first process of correlating a first measured value of a specific physical quantity at the target portion with a damage evaluation index obtained in advance, the damage evaluation index indicating a relationship between the specific physical quantity and a damage degree, the damage degree being calculated by dividing the usage elapsed time of the structural member by the total lifetime of the structural member, and calculating a damage degree corresponding to the first measured value; and a second process of measuring the specific physical quantity at least once at a position corresponding to the first measurement position in another time period having a different usage elapsed time from that of the first measurement; characterized in that in the first process, the total lifetime of the structural member is further calculated using the calculated damage degree and the usage elapsed time at the first measurement, in the second process, the second and subsequent measured values are correlated with damage degrees at the second and subsequent measurements, and the damage degrees at the second and subsequent measurements are further calculated using the usage elapsed time of the structural member at the second and subsequent measurements and the total lifetime calculated in the first process, and in that the damage evaluation method further comprises a third process of approximately calculating a new damage evaluation index based on a relationship between the first, second, and subsequent measured values and the damage degrees corresponding to the first, second, and subsequent measured values. a first process of correlating a first measured value of a specific physical quantity at the target portion with a damage evaluation index obtained in advance, the damage evaluation index indicating a relationship between the specific physical quantity and a damage degree, the damage degree being calculated by dividing the usage elapsed time of the structural member by the total lifetime of the structural member, and calculating a damage degree corresponding to the first measured value; and a second process of measuring the specific physical quantity at least once at a position corresponding to the first measurement position in another time period having a different usage elapsed time from that of the first measurement; characterized in that in the first process, the total lifetime of the structural member is further calculated using the calculated damage degree and the usage elapsed time at the first measurement, in the second process, the second and subsequent measured values are correlated with damage degrees at the second and subsequent measurements, and the damage degrees at the second and subsequent measurements are further calculated using the usage elapsed time of the structural member at the second and subsequent measurements and the total lifetime calculated in the first process, and in that the damage evaluation method further comprises a third process of approximately calculating a new damage evaluation index based on a relationship between the first, second, and subsequent measured values and the damage degrees corresponding to the first, second, and subsequent measured values.