A heat exchanger includes a plurality of tubes, a pair of header pipes, partition walls, an inlet connector block and an outlet connector block. The plurality of tubes have one ends connected to one header pipe and other ends connected to the other header pipe. The header pipe internally has a pipe-inside flow-through bore. The partition wall is internally formed in each header pipe to divide the pipe-inside flow-through bore into two regions. Each header pipe has a block connector bore opening at an outer side wall opposing to an area to which the tubes are connected and opening to the pipe-inside flow-though bore by cutting out a portion of the partition wall. The inlet connector block is connected to the block connector bore of one header pipe to admit coolant to flow in. The outlet connector block is connected to the block connector bore of the other header pipe to permit coolant to flow out.