  Magnetic phase diagrams of a tetragonal antiferromagnet TbB$_2$C$_2$ were
clarified by temperature and field dependence of magnetization. It is
noticeable that the N{\'e}el temperature in TbB$_2$C$_2$ is anomalously
enhanced with magnetic fields, in particular the enhancement reaches 13.5 K for
the ${<}1 1 0{>}$ direction at 10 T. The magnetization processes as well as the
phase diagrams are well interpreted assuming that there appear field-induced
antiferroquadrupolar ordered phases in TbB$_2$C$_2$. The phase diagrams of the
AFQ compounds in RB$_2$C$_2$ are systematically understood in terms of the
competition with AFQ and AFM interactions.
