  We calculate the large-q expansion of the second moment correlation length at
the first order phase transition point of the q-state Potts model in two
dimensions both in the ordered and disordered phases to order 21 in
$1/\sqrt{q}$. They coincide with each other to the third term of the series but
differ a little in higher orders. Numerically the ratio of the second moment
correlation length in the two phases is not far from unity in all region of
q>4. The ratio of the second moment correlation length to the standard
correlation length in the disordered phase is far from unity, which suggests
that the second largest and smaller eigenvalues of the transfer matrix form a
continuum spectrum not only in the large-q region but also in all the region of
q>4.
