A radiator includes an inlet line and an outlet line for a coolant to be cooled, and a plurality of flow passage units each being connected to the inlet and outlet lines respectively in parallel with adjacent flow passages with a space therebetween. Due to this configuration, it is possible to attain a high cooling capability with a small flow speed of the coolant, which is enabled by increasing the number of the flow passage units with respect to a certain flow rate passing through the inlet and outlet lines. This radiator can be employed for a cooling system together with a fan blowing air toward the flow passage units of the radiator, a coolant jacket for absorbing heat from a heat source such as a CPU, and a fluid pump for circulating the coolant between the coolant jacket and the radiator.