  The magnetic phase diagram of PrOs$_4$Sb$_{12}$ has been investigated by
specific heat measurements between 8 and 32 T. A new Schottky anomaly due to
excitations between two lowest crystalline-electric-field (CEF) singlets, has
been found for both $H \parallel (100)$ and $H \parallel (110)$ above the field
where the field-induced ordered phase (FIOP) is suppressed. The constructed
$H-T$ phase diagram shows weak magnetic anisotropy and implies a crossing of
the two CEF levels at about 8 - 9 T for both field directions. These results
provide an unambiguous evidence for the $\Gamma_1$ singlet being the CEF ground
state and suggest the level crossing (involving lowest CEF levels) as the
driving mechanism of FIOP.
