An oil cooler comprising: a plurality of flat tubes layered together with a clearance to one another in a layering direction, and configured to be mounted in a case, wherein the case allows cooling water to flow therein through the clearance in a longitudinal direction of the flat tubes, and wherein each flat tube allows working oil to flow therein; wherein: each flat tube includes: a first plate; a second plate including a periphery joined with a periphery of the first plate; a fin plate held between the first plate and the second plate; and an oil port having a cylindrical shape having a longitudinal axis extending substantially in the layering direction, and provided at first longitudinal end portions of the first plate and the second plate configured to receive inflow of cooling water; each flat tube is connected to one another via the oil port; the first plate is recessed to form a fin plate accommodation portion accommodating the fin plate, wherein the first longitudinal end portion of the first plate is outside of the fin plate accommodation portion in the longitudinal direction of the flat tube; the second plate has a substantially flat shape covering the fin plate accommodation portion of the first plate; the first longitudinal end portion of the first plate and the first longitudinal end portion of the second plate are joined together to form a thin portion of the flat tube; the oil port is provided at the thin portion of the flat tube and located adjacent to the fin plate accommodation portion of the first plate; each flat tube includes a guide wall at a lateral periphery thereof, wherein the guide wall faces the oil port substantially in a width direction of the flat tube, and projects in the layering direction; and the guide wall, the thin portion, and a lateral wall of the oil port form a nozzle portion of the flat tube to guide cooling water in the longitudinal direction of the flat tube. each flat tube includes: a first plate; a second plate including a periphery joined with a periphery of the first plate; a fin plate held between the first plate and the second plate; and an oil port having a cylindrical shape having a longitudinal axis extending substantially in the layering direction, and provided at first longitudinal end portions of the first plate and the second plate configured to receive inflow of cooling water; a first plate; a second plate including a periphery joined with a periphery of the first plate; a fin plate held between the first plate and the second plate; and an oil port having a cylindrical shape having a longitudinal axis extending substantially in the layering direction, and provided at first longitudinal end portions of the first plate and the second plate configured to receive inflow of cooling water; each flat tube is connected to one another via the oil port; the first plate is recessed to form a fin plate accommodation portion accommodating the fin plate, wherein the first longitudinal end portion of the first plate is outside of the fin plate accommodation portion in the longitudinal direction of the flat tube; the second plate has a substantially flat shape covering the fin plate accommodation portion of the first plate; the first longitudinal end portion of the first plate and the first longitudinal end portion of the second plate are joined together to form a thin portion of the flat tube; the oil port is provided at the thin portion of the flat tube and located adjacent to the fin plate accommodation portion of the first plate; each flat tube includes a guide wall at a lateral periphery thereof, wherein the guide wall faces the oil port substantially in a width direction of the flat tube, and projects in the layering direction; and the guide wall, the thin portion, and a lateral wall of the oil port form a nozzle portion of the flat tube to guide cooling water in the longitudinal direction of the flat tube.