Pressure dependence of the Yb valence in YbNiGe has been investigated up to 15.6 GPa at 300 K and up to 7.7 GPa at 17 K by means of x-ray absorption spectroscopy in partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb absorption edge. The Yb valence in YbNiGe at ambient pressure strongly fluctuates with the mean valence of at 300 K. The Yb valence rapidly changes toward a trivalent state with pressure up to 5 GPa, slowly increases up to 10 GPa, and then reaches a saturated value of at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; at ambient pressure and at 7.7 GPa. We found that the pressure-induced change in the intensity of a quadrupole component in the x-ray absorption spectra shows the same trend as the Yb valence in YbNiGe. In contrast to YbNiGe, the Yb valence in YbNiSi is nearly 3 at ambient pressure with almost no temperature dependence.