  We report on a cooperative phenomenon of ferromagnetism and unconventional
superconductivity (SC) in UGe$_2$ through the measurements of $^{73}$Ge
nuclear-quadrupole-resonance (NQR) under pressure ($P$). The NQR spectra
evidenced phase separation into ferromagnetic and paramagnetic phases in the
vicinity of $P_c\sim 1.5$ GPa, pointing to a first-order transition. The
measurements of nuclear-spin-lattice-relaxation-rate $1/T_1$ revealed that SC
emerges under the background of ferromagnetism, but not of the paramagnetic
phase.
