  We present the group-thoretical classification of gap functions in
superconductors coexisting with some magnetic order in non-symmorphic magnetic
space groups. Based on the weak-coupling BCS theory, we show that UCoGe-type
ferromagnetic superconductors must have horizontal line nodes on either $k_z=0$
or $\pm\pi/c$ plane. Moreover, it is likely that additional Weyl point nodes
exist at the axial point. On the other hand, in UPd$_2$Al$_3$-type
antiferromagnetic superconductors, gap functions with $A_g$ symmetry possess
horizontal line nodes in antiferromagnetic Brillouin zone boundary
perpendicular to $c$-axis. In other words, the conventional fully-gapped
$s$-wave superconductivity is forbidden in this type of antiferromagnetic
superconductors, irrelevant to the pairing mechanism, as long as the Fermi
surface crosses a zone boundary. UCoGe and UPd$_2$Al$_3$ are candidates for
unconventional superconductors possessing hidden symmetry-protected line nodes,
peculiar to non-symmorphic magnetic space groups.
