  Local magnetic field distribution B(r) in the mixed state of a boride
superconductor, YB6, is studied by muon spin rotation (muSR). A comparative
analysis using the modified London model and Ginzburg-Landau (GL) model
indicates that the GL model exhibits better agreement with muSR data at higher
fields, thereby demonstrating the importance of reproducing the field profile
near the vortex cores when the intervortex distance becomes closer to the GL
coherence length. The temperature and field dependence of magnetic penetration
depth ($\lambda$) does not show any hint of nonlocal effect nor of low-lying
quasiparticle excitation. This suggests that the strong coupling of electrons
to the rattling motion of Y ions in the boron cage suggested by bulk
measurements gives rise to a conventional superconductivity with isotropic
s-wave pairing. Taking account of the present result, a review is provided for
probing the anisotropy of superconducting order parameters by the slope of
$\lambda$ against field.
