  The response of vortex state to the magnetic field in Nb3Sn is probed using
muon spin rotation and small-angle neutron scattering. A transformation of
vortex structure between hexagonal and squared lattice is observed over a
relatively low field range of 2-3 Tesla. The gradual increase of the magnetic
penetration depth with increasing field provides microscopic evidence for
anisotropic (or multi-gapped) s-wave pairing suggested by the Raman scattering
experiment. This result renders need for careful examination on the difference
of electronic properties between Nb3Sn and V3Si.
