1. A fluid analyzing apparatus comprising: an advection step density analyzing unit which uses an advection equation of a mass conservation equation to calculate a density in an advection step; an advection step internal energy analyzing unit which uses an advection equation of an internal energy equation to calculate an internal energy in the advection step; an advection step pressure analyzing unit which calculates a pressure developing after advection based on the density and the internal energy in the advection step; an advection step velocity analyzing unit which uses an equation obtained by adding a term of pressure gradient developing before advection to an advection equation of a momentum conservation equation to calculate a velocity in the advection step; a nonadvection step small perturbation analyzing unit which calculates a small perturbation from the pressure developing after advection in a nonadvection step based on the density, the internal energy, the pressure, and the velocity after the advection step; a nonadvection step velocity analyzing unit which calculates a velocity developing after a unit time has elapsed based the small perturbations in the nonadvection step; a nonadvection step density analyzing unit which calculates a density developing after a unit time has elapsed based on the velocity developing after the unit time has elapsed and the density in the advection step; a nonadvection step internal energy analyzing unit which calculates an internal energy developing after the unit time has elapsed based on the velocity developing after the unit time has elapsed and the density after the advection step; and an iterative calculation control unit which iterates calculations performed from the advection step density analyzing unit to the nonadvection step internal energy analyzing unit in a predetermined order.