A processing solution tank of forming an agitating flow by flowing and circulating a processing solution in directions opposite to each other between the side of a liquid surface and the side of a tank bottom, in which a guide plate substantially of an arcuate shape is formed to an end of the tank situated at least to the downstream on the side of the liquid surface or the downstream on the side of the tank bottom in the agitating flow for reversing the flow downward to the tank bottom or upward to the liquid surface, and a swirl chamber having a swirl generating portion of a substantially arcuate face for capturing dusts is provided at the bottom of the tank, whereby high quality surface treatment, for example, electrodeposition coating can be conducted by uniformly stirring the processing solution so as not to cause backflow or turbulence in the processing solution tank while dipping a work on a conveyor in the processing solution.