In enzyme kinetics, the Quasi-Steady-State Assumption (QSSA) has been proposed for over 80 years, which plays a very important role in simplifying systems of equations derived from chemical reactions with enzymes. Five years ago, we proved that the QSSA is always true in the simplest model with the second elementary reaction irreversible, and called them as Quasi-Steady-State Laws. Thus, all conclusions based on QSSA have a solid foundation in irreversible case. However, the chemical reactions are not always so simple in many life processes. The second elementary reaction should be reversible in general, and the irreversible case is actually only an approximation. So it is more important and interesting to study the reversible case, and it has already attracted enzymologists for a long time. The basic assumption, i.e. QSSA in this general case has appeared in 1930. We proved this lasting over 80 years assumption in this paper.