  In this paper, defining a formula by $LDL^{T}$ decomposition for the minimal
type-I seesaw mechanism, we obtain conditions of $CP$ symmetry for the neutrino
mass matrix $m$ in an arbitrary basis. The conditions are found to be ${\rm
Re\,} (M_{22} a_{i} - M_{12} b_{i}) \, {\rm Im \,} ( M_{22} a_{j} - M_{12}
b_{j}) = - \det M \, {\rm Re\,} b_{i} \, {\rm Im \,} b_{j}$ or $ = - \det M \,
{\rm Im \,} b_{i} \, {\rm Re\,} b_{j}$ for the Yukawa matrix $Y_{ij} = (a_{j},
b_{j})$ and the right-handed neutrino mass matrix $M_{ij}$. In other words, the
real or imaginary part of $b_{i}$ must be proportional to the real or imaginary
part of the quantity $(M_{22} a_{i} - M_{12} b_{i})$.
