  Weak ferromagnetism of quasi-one-dimensional S = 1/2 antiferromagnet
BaCu$_2$Ge$_2$O$_7$ is studied by the magnetization measurement. The
spontaneous magnetization appears along the b axis. The local symmetry between
the in-chain nearest neighbor spins allows the presence of
Dzyaloshinskii-Moriya interaction, and the only possible spatial configuration
of the weak ferromagnetic moment per spin determines the sign of the
inter-chain interaction. A weak $a$-axis magnetic field can change the
direction of the magnetization to the $a$-axis direction, which shows that the
spin chain forms a weakly coupled weak-ferromagnetic chain system.
