  Identification of pairing mechanisms leading to the unconventional
superconductivity realized in copper-oxide, heavy-fermions, and organic
compounds is one of the most challenging issues in condensed-matter physics.
Clear evidence for an electron-phonon mechanism in conventional superconductors
is seen by the isotope effect on the superconducting transition temperatures
$T_{\rm SC}$, since isotopic substitution varies the phonon frequency without
affecting the electronic states. In unconventional superconductors, magnetic
fluctuations have been proposed to mediate superconductivity, and considerable
efforts have been made to unravel relationships between normal-state magnetic
fluctuations and superconductivity. Here, we show that characteristic
experimental results on the ferromagnetic (FM) superconductor UCoGe ($T_{\rm
Curie} \sim 2.5 $ K and $T_{\rm SC} \sim 0.6$ K) can be understood consistently
within a scenario of the spin-triplet superconductivity induced by FM spin
fluctuations. Temperature and angle dependencies of the upper critical magnetic
field of the superconductivity ($H_{c2}$) are calculated on the basis of the
above scenario by solving the Eliashberg equation. Calculated $H_{c2}$ well
agrees with the characteristic experimental results observed in UCoGe. This is
a first example that FM fluctuations are shown to be a pairing glue of
superconductivity.
