An interplanetary slow shock event was observed by Voyager 1 on 1 November 1980. The structure is identified to be a switch-off shock which is a special type of slow shocks by fitting the Rankine-Hugoniot relations. The shock has the following properties: (1) Observed parameters meet the R-H relations well. (2) Number density increases from the upstream region to the downstream region. On the contrary, total magnetic field strength decreases. (3) The slow-mode Mach number is greater than unity in the preshock state and less than unity in the postshock state. (4) The normal Alfvén-Mach number is almost equal to 1 in the preshock state; the tangential component of the downstream magnetic field is t"urned off," leaving only a minor quantity along the shock normal.