  We study a finite-time Carnot cycle of a weakly interacting gas which we can
regard as a nearly ideal gas in the limit of $T_\mathrm{h}-T_\mathrm{c}\to 0$
where $T_\mathrm{h}$ and $T_\mathrm{c}$ are the temperatures of the hot and
cold heat reservoirs, respectively. In this limit, we can assume that the cycle
is working in the linear-response regime and can calculate the Onsager
coefficients of this cycle analytically using the elementary molecular kinetic
theory. We reveal that these Onsager coefficients satisfy the so-called
tight-coupling condition and this fact explains why the efficiency at the
maximal power $\eta_\mathrm{max}$ of this cycle can attain the Curzon-Ahlborn
efficiency from the viewpoint of the linear-response theory.
