  Let $(\Omega,{\mathcal F},P)$ be a probability space and $L^0({\mathcal F})$
the algebra of equivalence classes of real-valued random variables defined on
$(\Omega,{\mathcal F},P)$. A left module $M$ over the algebra $L^0({\mathcal
F})$(briefly, an $L^0({\mathcal F})$-module) is said to be regular if $x=y$ for
any given two elements $x$ and $y$ in $M$ such that there exists a countable
partition $\{A_n,n\in \mathbb N\}$ of $\Omega$ to $\mathcal F$ such that
${\tilde I}_{A_n}\cdot x={\tilde I}_{A_n}\cdot y$ for each $n\in \mathbb N$,
where $I_{A_n}$ is the characteristic function of $A_n$ and ${\tilde I}_{A_n}$
its equivalence class. The purpose of this paper is to establish the
fundamental theorem of affine geometry in regular $L^0({\mathcal F})$-modules:
let $V$ and $V^\prime$ be two regular $L^0({\mathcal F})$-modules such that $V$
contains a free $L^0({\mathcal F})$-submodule of rank $2$, if $T:V\to V^\prime$
is stable and invertible and maps each $L^0$-line segment to an $L^0$-line
segment, then $T$ must be $L^0$-affine.
