Technique is provided that enables detection of changes in the condition of a power generation system that generates fluctuating power outputs even in a normal condition thereof. A power generation system management apparatus has: range information management means for managing a condition determination power output range corresponding to a range in which a power output at a predetermined measurement point of the power generation system is to be included with a predetermined probability or higher, the condition determination power output range being determined based on a standard deviation of a plurality of model construction power output values, which is calculated using a representative value of the power output at the measurement point and the model construction power output values, the representative value being calculated from a pseudo system model that is created with a non-parametric method using the plurality of model construction power output values.