  It appears natural to consider the four dimensional relativistic massive
field as a five dimensional massless field. If the fifth coordinate is
interpreted as the proper time, then the fifth moment can be understood as the
rest mass. After introducing the rest mass operator, we define the mass
eigenstate and the nonmass eigenstate. The general equations of spin 0, spin
1/2 and spin 1 fields are obtained respectively. It is shown that the
Klein-Gordon equation, the Dirac equation and the Proca equation describe the
mass eigenstates only. The rest mass of spin 1/2 field and the rest mass
squared of Boson fields are calculated. The U(1) gauge field that couples to
the nonmass eigenstates is studied carefully, whose gauge boson can be massive.
