  In this paper, we study a combined system of a Fokker-Planck (FP) equation
for $m^{t,\mu}$ with initial $(t,\mu)\in[0,T]\times L^2(\mathbb{R}^d)$, and a
stochastic differential equation for $X^{t,x,\mu}$ with initial
$(t,x)\in[0,T]\times \mathbb{R}^d$, whose coefficients depend on the solution
of FP equation. We develop a combined probabilistic and analytical method to
explore the regularity of the functional
$V(t,x,\mu)=\mathbb{E}[\Phi(X^{t,x,\mu}_T,m^{t,\mu}(T,\cdot))]$. Our main
result states that, under a non-degenerate condition and appropriate regularity
assumptions on the coefficients, the function $V$ is the unique classical
solution of a nonlocal partial differential equation of mean-field type. The
proof depends heavily on the differential properties of the flow $\mu\mapsto
(m^{t,\mu}, X^{t,x,\mu})$ over $\mu\in L^2(\mathbb{R}^d)$. We also give an
example to illustrate the role of our main result. Finally, we give a
discussion on the $L^1$ choice case in the initial $\mu$.
