Mixing in a continuous vibrating mixer named "VIBRO MIXER(R)" has been observed with the aid of a visualization technique and a tracer response technique. The mixer consists of several mixing units having a spiral impeller going up and down. It has been found that homogeneous and rapid mixing can be obtained owing to the spiral structure of the impeller and the holes opened in the impeller. On the other hand, mixing between units is inhibited by a partition plate. Owing to the coupling of these two mixing effects, the mixer is regarded as a plug-flow type mixing device. When the mixer is operated under sufficiently high mixing Reynolds number and space velocity, the mixing can be described as a tanks-in-series model having the same number of tanks as the mixing unit.