We study the soft mode along the critical line in the phase diagram with the tricritical point, using the Nambu–Jona-Lasinio model. At the critical point with finite quark mass, the ordering density becomes a linear combination of the scalar, quark number and energy densities, and their susceptibilities diverge with the same exponent. Based on the conservation law, it is argued that the divergent susuceptibility of a conserved density must be accompanied by a critically–slowing hydrodynamic mode. The shift of the soft mode from the sigma meson to the hydrodynamic mode occurs at the tricritical point on the critical line.