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 $Bsim 0.1$~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{cosbeta} > 27 $s (95 % C.L.) for any $B$ in the interval ($0.06div0.25$) G.
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