The character change of a superfluid state due to the variation of the
attractive force is investigated in the relativistic framework with a massive
fermion. Two crossovers are found. One is a crossover from the usual BCS state
to the Bose-Einstein condensation (BEC) of bound fermion pairs. The other is
from the BEC to the relativistic Bose-Einstein condensation (RBEC) of nearly
massless bound pairs where antiparticles as well as particles dominate the
thermodynamics. Possible realization of the BEC and RBEC states in the quark
matter is also pointed out.Comment: 5 pages, 1 figure, revtex4; (v2) text has been clarified, references
updated; (v3) final version to appear in Phys. Rev.
