1.  Field of the Invention
The present invention relates to a fuel cell including a membrane electrode assembly and separators sandwiching the membrane electrode assembly.  The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. 
2.  Description of the Related Art
For example, a solid polymer fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane.  The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly.  Each of the anode and the cathode is made of electrode catalyst and porous carbon.  The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. 
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode.  The air or the like (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode.  The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons.  The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. 
In operation of the fuel cell, typically, a voltage detection device is used to detect a cell voltage at the time of power generation for detecting whether the desired power generation performance is achieved or not.  In relation to this operation, Japanese Laid-Open Patent Publication No.  11-339828 discloses a fuel cell stack equipped with a cell voltage measuring terminal. 
According to the disclosure of Japanese Laid-Open Patent Publication No.  11-339828, the fuel cell stack includes a metal separator 1 as shown in FIG.  6.  The separator 1 has an air supply passage 2a, a cooling water passage 3, a hydrogen supply passage 4a, an air discharge passage 2b, and a hydrogen discharge passage 4b.  For example, a pin shaped voltage measuring terminal 6 is welded to one end surface 5 of the separator 1 by projection welding. 
However, in the fuel cell stack of Japanese Laid-Open Patent Publication No.  11-339828, the pin shaped voltage measuring terminal 6 protrudes from the end surface 5 of the separator 1.  Therefore, at the time of assembling the fuel cell or the fuel cell stack, the voltage measuring terminal 6 is deformed easily.  Thus, the quality of the separator 1 lowered, and the number of processes required for the assembling operation is large.  In particular, when the separator 1 is thin, the diameter of the voltage measuring terminal 6 is significantly small.  The voltage measuring terminal 6 is deformed easily, and may be damaged (broken) undesirably.