Soft budget constraint refers to the situation where an economic entity expects to obtain economic assistance when in financial difficulties. During the past decade, a sizable literature has accumulated explaining the causes and consequences of the soft budget constraint. Many of the theories have traced soft budget constraint on enterprises to that on banks. However, why do banks often face soft budget constraint? How to mitigate the resulting problems? In this paper, we first show that owing to their special financial structure, banks as market institutions intrinsically face hard budget constraint and nevertheless remain stable and effective. Since banks’ finance mostly comes from deposits, it is very difficult for banks to be refinanced when their investment projects are unsuccessful due to the sequential service arrangement for bank deposits. This limitation hardens the budget constraint on banks and disciplines bankers’ investment decisions. However, the advent of instantaneous-social-welfareminded modern governments, which have both the resources and the incentives to bail out failing banks, gives rise to the soft budget constraint of banks. This causes bankers’ moral hazard problems. As an institutional solution to the resulting banking instabilities, banking regulation emerged in order to restrict banks’ investment decisions. We provide historical evidence on the genesis and symptoms of, and institutional solution to the soft budget constraint of banks over the past six hundred years to support our theory. We also conduct contemporary econometric analysis to show how the lack of government commitment to a hard budget constraint gives rise to a strict banking regulation. We further explore the predictions of our theory in the paper.