The aim of this paper is to prove a functional equation for a local zeta function attached to the minimal spherical series for a class of real reductive symmetric spaces. These symmetric spaces are obtained as follows. We consider a graded simple real Lie algebra $\widetilde{\mathfrak g}$ of the form $\widetilde{\mathfrak g}=V^-\oplus \mathfrak g\oplus V^+$, where $[\mathfrak g,V^+]\subset V^+$, $[\mathfrak g,V^-]\subset V^-$ and $[V^-,V^+]\subset \mathfrak g$. If the graded algebra is regular, then a suitable group $G$ with Lie algebra $\mathfrak g$ has a finite number of open orbits in $V^+$, each of them is a realization of a symmetric space $G\slash H_p$. The functional equation gives a matrix relation between the local zeta functions associated to $H_p$-invariant distributions vectors for the same minimal spherical representation of $G$. This is a generalization of the functional equation obtained by Godement} and Jacquet for the local zeta function attached to a coefficient of a representation of $GL(n,\mathbb R)$.