  We report electrical resistivity, ac magnetic susceptibility and X-ray
absorption spectroscopy measurements of intermediate valence YbNi$_{3}$Ga$_{9}$
under pressure and magnetic field. We have revealed a characteristic
pressure-induced Yb valence crossover within the temperature-pressure phase
diagram, and a first-order metamagnetic transition is found below $P_{\rm c}$
$\sim$ 9 GPa where the system undergoes a pressure-induced antiferromagnetic
transition. As a possible origin of the metamagnetic behavior, a critical
valence fluctuation emerging near the critical point of the first-order valence
transition is discussed on the basis of the temperature-field-pressure phase
diagram.
