  Causality is studied in the covariant formulation of free string field theory
(SFT). We find that, though the string field in the covariant formulation is a
functional of the ghost coordinates as well as the space-time coordinate and
the latter contains the time-like oscillators with negative norm, the condition
for the commutator of two open string fields to vanish is simply given by
$\int_0^\pi d\sigma\left(\Delta X^\mu(\sigma)\right)^2 >0$, which is the same
condition as in the light-cone gauge SFT. For closed SFT, the corresponding
condition is given in a form which is manifestly invariant under the rigid
shifts of the $\sigma$ parameters of the two string fields.
