1. A non-transitory computer-readable recording medium in which a thermal hydraulic simulation program that carries out a simulation of a velocity field and/or a temperature field is stored, the program instructing a computer to execute: carrying out a first simulation of the velocity field and the temperature field, using a velocity-field analyzing model and a temperature-field analyzing model having respective first degrees of freedom; determining levels of reducing the first degrees of freedom on the basis of the result of the first simulation; calculating a velocity-field dimension transformation matrix and a temperature-field dimension transformation matrix that respectively reduce the degrees of freedom of the velocity-field analyzing model and the temperature-field analyzing model to respective second degrees of freedom by the levels of reducing determined; and converting the velocity-field analyzing model and the temperature-field analyzing model into reduced degree-of-freedom models having the second degrees of freedom using the velocity-field dimension transformation matrix and the temperature-field dimension transformation matrix calculated; and carrying out a second simulation of the velocity field and the temperature field using the reduced degree-of-freedom models.