  We show a possibility that strong ``magnetic field'' $\sim 10^{15} $G is
produced by color ferromagnetic quark matter in neutron stars. In the quark
matter a color magnetic field is generated spontaneously owing to Savvidy
mechanism and a gluon condensate arises for the stabilization of the field.
Since the quark matter is electrically charged in the neutron stars, the
rotation of the quarks around the color magnetic field produces the strong
``magnetic field''.
