  Shape evolution of Zr nuclei are investigated by the axial Hartree-Fock (HF)
calculations using the semi-realistic interaction M3Y-P6, with focusing on
roles of the tensor force. Deformation at $N\approx 40$ is reproduced, which
has not been easy to describe within the self-consistent mean-field
calculations. The spherical shape is obtained in $46\leq N\leq 56$, and the
prolate deformation is predicted in $58\leq N\leq 72$, while the shape switches
to oblate at $N=74$. The sphericity returns at $N=80$ and $82$. The deformation
in $60\lesssim N\lesssim 70$ resolves the discrepancy in the previous
magic-number prediction based on the spherical mean-field calculations [Prog.
Theor. Exp. Phys. \textbf{2014}, 033D02]. It is found that the deformation at
$N\approx 40$ takes place owing to the tensor force with a good balance. The
tensor-force effects significantly depend on the configurations, and are
pointed out to be conspicuous when the unique-parity orbit (e.g. $n0h_{11/2}$)
is present near the Fermi energy, delaying deformation. These effects are
crucial for the magicity at $N=56$ and for the predicted shape change at $N=74$
and $80$.
