    var latlngs= [];
    var polyline = null;
    window.onload = function() {
      // ......
      mymap.on('click', function(e) {
        latlngs.push(e.latlng);
        if (latlngs == null || latlngs.count == 1) {
          return; // ̃NbNɂ͐`悵ȂB
        }
        if (polyline != null) {
          polyline.remove(); // ĕ`̑Oɕ`ς݂̒B
        }
        polyline = L.polyline(latlngs, {color: 'red'}).addTo(mymap);
      });
    }