1. A maintenance system comprising: a plurality of measurement targets; a measurement unit configured to periodically measure a usage status of a measurement target, and to measure a test item of the measurement target at an arbitrary timing according to a test schedule having a plurality of test items, measured data of the test item being a test result; a first storage unit configured to store the usage status and the test result in time series; a second storage unit configured to store statistic information representing a relationship between the usage status and the test result; a probability calculation unit configured to calculate a first evaluation value representing a load of the measurement target at a recent timing and a second evaluation value representing a load of the measurement target at the arbitrary timing, based on the usage status stored in the first storage unit, and to calculate a specific probability of each test item, based on the statistic information, the first evaluation value and the second evaluation value; and a test schedule creation unit configured to create a new test schedule by selecting from the plurality of test items, based on the specific probability.