  We here demonstrate that the superfluid 3He-B under a magnetic field in a
particular direction stays topological due to a discrete symmetry, that is, in
a symmetry protected topological order. Due to the symmetry protected
topological order, helical surface Majorana fermions in the B phase remain
gapless and their Ising spin character persists. We unveil that the competition
between the Zeeman magnetic field and dipole interaction involves anomalous
quantum phase transition where topological phase transition takes place
together with spontaneous breaking of symmetry. Based on the quasiclassical
theory, we illustrate that the phase transition is accompanied by anisotropic
quantum criticality of spin susceptibilities on the surface, which is
detectable in NMR experiments.
