  We report on the magnetic penetration depth, lambda, in a type II
superconductor NbB_{2+x} determined by muon spin rotation method. We show in
the sample with x=0.1 that lambda at 2.0 K is independent of an applied
magnetic field. This suggests that the superconducting order parameter in
NbB_{2+x} is isotropic, as expected for conventional BCS superconductors.
Meanwhile, the superconducting carrier density (proportional to lambda^{-2})
exhibits an interesting tendency of increase with increasing T_c (where T_c
varies with x). Possible origin of such behavior is discussed in comparison
with the case of exotic superconductors.
