  Using the direct method, we prove the generalised Hyers-Ulam stability of the
following functional equation \begin{equation} \phi(x+y, z+w)+\phi(x-y, z-w)-2
\phi(x, z)-2 \phi(x, w)=0 \end{equation} in modular space satisfying the Fatou
property or $\Delta_2$-condition.
