A heat exchanger is disclosed including a first tube, a second tube, a first inlet-connecting block, a first outlet-connecting block, a second inlet-connecting block, a second outlet-connecting block and a terminal connecting block. The first tube internally has a first flow passage through which first coolant flows. The second tube internally has a second flow passage through which second coolant flows. The first tube is connected to the first inlet-connecting block and the first outlet-connecting block. The second tube is classified into an inflow tube group and an outflow tube group. Tubes that belong to the inflow tube group are connected to the second inlet-connecting block and the terminal connecting block. Tubes that belong to the outflow tube group are connected to and the terminal connecting block and the second outlet-connecting block. The first and second tubes are disposed such that the first and second flow passages are substantially perpendicular to one another.