A heat exchanger includes a flow passage pipe that has a flat shape having a predetermined thickness, a heat medium flowing in the flow passage pipe, the heat medium exchanging heat with a heat exchange target, and an inner fin located inside the flow passage pipe. The inner fin includes a wavy fin that partitions a main passage into multiple partitioned passages, and a guide wall connected to the wavy fin. An x-direction is a lengthwise direction of the flow passage pipe, a z-direction is a thickness direction of the flow passage pipe, and a y-direction is a direction perpendicular to both the x-direction and the z-direction. The wavy fin includes a first convex portion convex to a first side in the y-direction, and a second convex portion convex to a second side in the y-direction. The wavy fin has an opening portion through which two partitioned passages adjacent to each other communicate with each other. The guide wall protrudes from the wavy fin into the partitioned passage. The heat exchanger is capable of enhancing a heat transfer and improving a thermal performance of the heat exchanger.