  Present probes do not exclude that the neutron ($n$) oscillation into mirror
neutron ($n'$), a sterile state exactly degenerate in mass with the neutron,
can be a very fast process, in fact faster than the neutron decay itself. This
process is sensitive to the magnetic field. Namely, if the mirror magnetic
field $\vec{B}'$ exists at the Earth, $n-n'$ oscillation probability can be
suppressed or resonantly amplified by the applied magnetic field $\vec{B}$,
depending on its strength and on the angle $\beta$ between $\vec{B}$ and
$\vec{B}'$. We present the results of ultra-cold neutron storage measurements
aiming to check the anomalies observed in previous experiments which could be a
signal for $n-n'$ oscillation in the presence of mirror magnetic field $B'\sim
0.1$~G. Analyzing the experimental data on neutron loses, we obtain a new lower
limit on $n-n'$ oscillation time $\tau_{nn'} > 17$ s (95 % C.L.) for any $B'$
between 0.08 and 0.17 G, and $\tau_{nn'}/\sqrt{\cos\beta} > 27 $s (95 % C.L.)
for any $B'$ in the interval ($0.06\div0.25$) G.
