An immersion-type surface treatment tank includes a treatment tank body including: a single tank internal space elongated in a plan view, and a nozzle that ejects an electrodeposition paint into the tank internal space. The treatment tank body includes: a first tank inner side surface extending along a longitudinal direction of the tank internal space; a second tank inner side surface facing the first tank inner side surface and extending along the longitudinal direction; and a rectifying plate that is formed halfway in the longitudinal direction and changes a flow direction of the electrodeposition paint such that the electrodeposition paint flowing horizontally along the first tank inner side surface is directed toward the second tank inner side surface. The rectifying plate changes the flow direction of the electrodeposition paint, thereby forming, in the tank internal space, at least two horizontal swirl flows adjacent to each other in the longitudinal direction.