  A complete structure theorem of Sally modules of $\fkm$-primary ideals $I$ in
a Cohen-Macaulay local ring $(A, \m)$ satisfying the equality
$\e_1(I)=\e_0(I)-\ell_A(A/I)+1$ is given, where $\e_0(I)$ and $\e_1(I)$ denote
the first two Hilbert coefficients of $I$.
