  The equal-time pairing correlation function of the two-dimensional t-J model
on a square lattice is studied using a high-temperature expansion method. The
sum of the pairing correlation, its spatial dependence, and the correlation
length are obtained as functions of temperature down to $T \simeq 0.2 t$. By
comparison of single-particle contributions in the correlation functions, we
find an effective attractive interaction between quasi-particles in
$d_{x^2-y^2}$-wave pairings. It is shown that d-wave correlation grows rapidly
at low temperatures for 0.5 < n < 0.9, with n being the electron density. The
temperature for this growth is roughly scaled by J/2. This is in sharp contrast
to the Hubbard model in a weak or intermediate coupling region, where there is
no numerical evidence of superconductivity.
