A fin structure, a heat-transfer tube and a heat exchanger are formed of plate fins housed in a heat-transfer tube and have an excellent cooling efficiency by making the distribution and flow velocity of a flow uniform and by promoting an efficient heat-exchanging action. The fin structure includes plate fins housed in a heat-transfer tube and having a square section and a free shape in the longitudinal direction for dividing a passage for a fluid composed of a cooled medium or a cooling medium to flow in the heat-transfer tube, into a plurality of small passages. In the fin structure, notches, through holes, raised portions, ridges and/or troughs are formed in the sides or the upper or lower walls of the plate fins. The heat-transfer tube has the fin structure housed therein. The heat exchanger has the heat-transfer tube assembled therein.