A performance degradation diagnosis system for a refrigeration cycle apparatus that includes a refrigerant circuit including a compressor, a heat-source-side heat exchanger, and a use-side heat exchanger, the refrigeration cycle apparatus further including a heat-source-side fan configured to supply air to the heat-source-side heat exchanger and an expansion valve disposed between the heat-source-side heat exchanger and the use-side heat exchanger in the refrigerant circuit and configured to decompress refrigerant flowing in the refrigerant circuit, the performance degradation diagnosis system comprising: a determining unit configured to determine, based on an index indicating an operation state of the refrigeration cycle apparatus, performance degradation of the refrigeration cycle apparatus with respect to each of a plurality of performance degradation factors of the refrigeration cycle apparatus, the performance degradation factors including a decrease in an amount of refrigerant, and a control unit configured to, in a case where the determining unit determines performance degradation of the refrigeration cycle apparatus with respect to each of the performance degradation factors, grasp an operation condition of the refrigeration cycle apparatus which is operating, and in a case where the operation condition of the refrigeration cycle apparatus is not suitable for determining performance degradation of the refrigeration cycle apparatus with respect to the performance degradation factor, control the operation condition of the refrigeration cycle apparatus so that the operation condition of the refrigeration cycle apparatus becomes an appropriate operation condition, which is suitable for determining performance degradation of the refrigeration cycle apparatus with respect to the performance degradation factor, wherein the control unit is configured to, in a case where the determining unit determines performance degradation of the refrigeration cycle apparatus due to the decrease in the amount of refrigerant, and a degree of subcooling in a refrigeration cycle of the refrigeration cycle apparatus which is operating has a value smaller than a first predetermined value, cause the refrigeration cycle apparatus to execute at least one of an operation of increasing the number of rotations of the compressor, an operation of decreasing the number of rotations of the heat-source-side fan, and an operation of decreasing an opening degree of the expansion valve, to control the operation condition of the refrigeration cycle apparatus to the appropriate operation condition. a determining unit configured to determine, based on an index indicating an operation state of the refrigeration cycle apparatus, performance degradation of the refrigeration cycle apparatus with respect to each of a plurality of performance degradation factors of the refrigeration cycle apparatus, the performance degradation factors including a decrease in an amount of refrigerant, and a control unit configured to, in a case where the determining unit determines performance degradation of the refrigeration cycle apparatus with respect to each of the performance degradation factors, grasp an operation condition of the refrigeration cycle apparatus which is operating, and in a case where the operation condition of the refrigeration cycle apparatus is not suitable for determining performance degradation of the refrigeration cycle apparatus with respect to the performance degradation factor, control the operation condition of the refrigeration cycle apparatus so that the operation condition of the refrigeration cycle apparatus becomes an appropriate operation condition, which is suitable for determining performance degradation of the refrigeration cycle apparatus with respect to the performance degradation factor, wherein the control unit is configured to, in a case where the determining unit determines performance degradation of the refrigeration cycle apparatus due to the decrease in the amount of refrigerant, and a degree of subcooling in a refrigeration cycle of the refrigeration cycle apparatus which is operating has a value smaller than a first predetermined value, cause the refrigeration cycle apparatus to execute at least one of an operation of increasing the number of rotations of the compressor, an operation of decreasing the number of rotations of the heat-source-side fan, and an operation of decreasing an opening degree of the expansion valve, to control the operation condition of the refrigeration cycle apparatus to the appropriate operation condition.