  We study the interplay of the ferromagnetic (FM) state and the p-wave
superconducting (SC) state observed in several materials such as UCoGe and
URhGe in a totally nonperturbative manner. To this end, we introduce a lattice
Ginzburg-Landau model that is a genuine generalization of the phenomenological
Ginzburg-Landau theory proposed previously in the continuum and also a
counterpart of the lattice gauge-Higgs model for the s-wave SC transition, and
study it numerically by Monte-Carlo simulations. The obtained phase diagram has
qualitatively the same structure as that of UCoGe in the region where the two
transition temperatrures satisfy $T_{\rm FM}>T_{\rm SC}$. For $T_{\rm
FM}/T_{\rm SC} < 0.7$, we find that the coexisting region of FM and SC orders
appears only near the surface of the lattice, which describes an inhomogeneous
FMSC coexisting state.
