  We have investigated the suppression of the superconducting transition
temperature, Tc, with an increase of the residual resistivity, \rho_0, through
the substitution of M (M = Co, Ni, Zn) for Fe in Fe1-yMySe0.3Te0.7 single
crystals, in order to clarify the symmetry of the superconducting gap in
FeSe1-xTex. Small, large and very small suppression of Tc have been observed
through the Co, Ni and Zn substitution, respectively. The magnitude of the
suppression rate is hardly explained in terms of the pair-breaking effect due
to potential scattering calculated based on the Abrikosov-Gor'kov theory, even
if errors in the estimation of the carrier concentration, effective mass and
\rho_0 were taken into account. Accordingly, these results suggest that the
superconducting symmetry is the s++-wave in FeSe1-xTex.
