  We show that the field dependence of the magnetic penetration depth
($\lambda$), for which muon spin rotation ($\mu$SR) is an excellent microscopic
probe, provides useful information on the degree of anisotropy for the
superconducting order parameter. In type II superconductors associated with
anisotropic order parameters, $\lambda$ is sensitive to the quasiparticle
excitation induced by the Doppler shift due to a supercurrent around magnetic
vortices. The presence of such low-energy excitations manifests itself in the
non-zero slope of $\lambda$ against an external magnetic field. We review
recent results on the field dependence of $\lambda$ obtained from the
application of $\mu$SR to novel superconductors that exhibit unconventional
characters associated with the anisotropic order parameter.
