  The property of the spectrum of large-scale magnetic fields generated due to
the breaking of the conformal invariance of the Maxwell theory through some
mechanism in inflationary cosmology is studied. It is shown that the spectrum
of the generated magnetic fields should not be perfectly scale-invariant but be
slightly red so that the amplitude of large-scale magnetic fields can be
stronger than $\sim 10^{-12}$G at the present time. This analysis is performed
by assuming the absence of amplification due to the late-time action of some
dynamo (or similar) mechanism.
