  Phenomenological Landau analysis shows that the properties of ordered phases
in some skutterudites are consistently accounted for by a scalar order
parameter which preserves the cubic symmetry, even in the ordered phase. A
universal value is found for the anisotropy ratio of the transition temperature
in a magnetic field, homogeneous magnetization, and induced staggered
magnetization. The difference in magnetic behavior between PrFe$_4$P$_{12}$ and
PrRu$_4$P$_{12}$ near their phase transitions is explained within a single
framework. For the low-field phase of PrFe$_4$P$_{12}$, the scalar order with
the $\Gamma_{1g}$ symmetry can explain (i) the absence of field induced dipoles
perpendicular to the magnetic field, (ii) isotropic magnetic susceptibility in
the ordered phase, (iii) the field angle dependence of the transition
temperature, and (iv) the splitting pattern of the $^{31}$P nuclear magnetic
resonance (NMR) spectra. It is proposed how the order parameter in
SmRu$_4$P$_{12}$ is identified by NMR analysis of a single crystal.
