  The anisotropy and temperature dependence of the magnetic field penetration
depth in the iron-based superconductor KFe$_2$As$_2$ have been measured down to
20 mK by means of muon spin rotation. We have observed that the flux-line
lattice forms triangular symmetry when we applied the field parallel to c-axis,
suggesting the interaction between vortices should be isotropic. The
temperature dependence of the penetration depth observd with an applied field
parallel and perpendicular to c-axis are different. These results can be
accounted for by a superconducting gap function with a horizontal line node in
the basal plane.
