An insulation deterioration determination method executed by a computer device including a CPU that is adapted to determine a degree of insulation deterioration occurring in a power transmission cable and a storage device, the method comprising: a first step of sequentially generating a plurality of distribution patterns (T' 1,..., T' 16) by standardizing a distribution pattern of a combination of charge amounts and phase angles of each one of a plurality of insulation deterioration pulses occurring in a period corresponding to several applied voltage cycles of an applied voltage signal, a second step of executing a first classification process in which the plurality of distribution patterns (T' 1,..., T' 16) is classified into at least one class on the basis of a standard distribution pattern for each class, wherein each class is adapted to indicate a deterioration stage, wherein each class is determined by comparing the plurality of distribution patterns (T' 1,..., T' 16) with the standard distribution pattern which is created in advance and stored in the storage device, and wherein a second classification process is executed, in which the plurality of the distribution patterns (T' 1,..., T' 16) is classified so that the distribution patterns (T' 1,..., T' 16) are grouped into at least one cluster (A1, A2) by using an algorithm such as a principal component analysis method or k-means clustering; a third step of evaluating a classification result by the first classification process on the basis of a classification result by the second classification process; and a fourth step of determining the degree of progress of the insulation deterioration occurring in the power transmission cable on the basis of an evaluation result in the third step. a first step of sequentially generating a plurality of distribution patterns (T' 1,..., T' 16) by standardizing a distribution pattern of a combination of charge amounts and phase angles of each one of a plurality of insulation deterioration pulses occurring in a period corresponding to several applied voltage cycles of an applied voltage signal, a second step of executing a first classification process in which the plurality of distribution patterns (T' 1,..., T' 16) is classified into at least one class on the basis of a standard distribution pattern for each class, wherein each class is adapted to indicate a deterioration stage, wherein each class is determined by comparing the plurality of distribution patterns (T' 1,..., T' 16) with the standard distribution pattern which is created in advance and stored in the storage device, and wherein a second classification process is executed, in which the plurality of the distribution patterns (T' 1,..., T' 16) is classified so that the distribution patterns (T' 1,..., T' 16) are grouped into at least one cluster (A1, A2) by using an algorithm such as a principal component analysis method or k-means clustering; a third step of evaluating a classification result by the first classification process on the basis of a classification result by the second classification process; and a fourth step of determining the degree of progress of the insulation deterioration occurring in the power transmission cable on the basis of an evaluation result in the third step.