  We study the spontaneous $CP$ violation through the stabilization of the
modulus $\tau$ in modular invariant flavor models. The $CP$-invaraiant
potentential has the minimum only at ${\rm Re}[\tau] = 0$ or 1/2. From this
prediction, we study $CP$ violation in modular invariant flavor models. The
physical $CP$ phase is vanishing. The important point for the $CP$ conservation
is the $T$ transformation in the modular symmetry. One needs the violation of
$T$ symmetry to realize the spontaneous $CP$ violation.
