As is conventionally known, in order to efficiently generate electricity with a polymer electrolyte fuel cell, it is desirable to place the inside of cells in a moderately wet condition and to avoid an excessive or deficient moisture content in the fuel cell. For example, a technique described in Patent Document 1 is known as a technique for controlling a moisture content in a cell of a fuel cell. With the technique according to Patent Document 1, a water content in a cell is adjusted by reducing pressure of gas (for example, hydrogen gas) at an anode electrode to increase water displacement from a cathode electrode to the anode electrode.    Patent document 1: Patent Publication JP-A-2008-103137
However, in reality, there are cases where reducing the gas pressure at the anode electrode is insufficient for achieving adjustment to a desired water content. For example, when gas pressure is drastically reduced in an attempt to increase water content, a hydrogen gas partial pressure at the anode electrode may decrease and an electrical generation output of the fuel cell may decrease. In addition, when gas pressure at the anode electrode side is drastically reduced, a risk arises that discharge of offgas or the like from the anode electrode to the outside using a difference in pressure cannot be sufficiently performed.
The present invention has been made in consideration of the points described above and an object thereof is to provide a method of controlling a water content of a fuel cell and a fuel cell system which are capable of properly adjusting a water content in a cell of a fuel cell in accordance with a wide variety of conditions.