  The magnetic phase diagram for antiferro-quadrupole (AFQ) ordering in
tetragonal HoB$_2$C$_2$ has been investigated by measurements of elastic
constants $C_{11}$, $C_{44}$ and $C_{66}$ in fields along the basal $x$-$y$
plane as well as the principal [001]-axis. The hybrid magnet (GAMA) in Tsukuba
Magnetic Laboratory was employed for high field measurements up to 30 T. The
AFQ phase is no longer observed above 26.3 T along the principal [001] axis in
contrast to the relatively small critical field of 3.9 T in fields applied
along the basal [110] axis. The quadrupolar intersite interaction of $O_{xy}$
and/or $O_2^2$ is consistent with the anisotropy in the magnetic phase diagram
of the AFQ phase in HoB$_2$C$_2$.
