  We performed high-magnetic-field ultrasonic experiments on YbB$_{12}$ up to
59 T to investigate the valence fluctuations in Yb ions. In zero field, the
longitudinal elastic constant $C_{11}$, the transverse elastic constants
$C_{44}$ and $\left( C_{11} - C_{12} \right)/2$, and the bulk modulus
$C_\mathrm{B}$ show a hardening with a change of curvature at around 35 K
indicating a small contribution of valence fluctuations to the elastic
constants. When high magnetic fields are applied at low temperatures,
$C_\mathrm{B}$ exhibits a softening above a field-induced insulator-metal
transition signaling field-induced valence fluctuations. Furthermore, at
elevated temperatures, the field-induced softening of $C_\mathrm{B}$ takes
place at even lower fields and $C_\mathrm{B}$ decreases continuously with
field. Our analysis using the multipole susceptibility based on a two-band
model reveals that the softening of $C_\mathrm{B}$ originates from the
enhancement of multipole-strain interaction in addition to the decrease of the
insulator energy gap. This analysis indicates that field-induced valence
fluctuations of Yb cause the instability of the bulk modulus $C_\mathrm{B}$.
