  Using heavy-baryon chiral perturbation theory, we have calculated all the
diagrams up to two-loop order which contribute to the S-wave pion self-energy
in symmetric nuclear matter. Some subtleties related to the definition of pion
fields are discussed. The in-medium pion mass is turned out to be increased by
only (5 - 6) per cents in normal nuclear matter density, without any off-shell
ambiguity.
