A cooling system controller controls a cooling system at least having a coolant pump (3) configured to supply a coolant to an internal combustion engine, a heat radiator (9) configured to cool the coolant, and an exhaust heat recovery system (6) configured to perform in heat exchange between the exhaust gas and the coolant. In addition, the cooling system controller (12) has a unit configured to control the coolant flow rate, a unit (13) configured to obtain a coolant temperature or pressure of the internal coolant passage of the engine, and a unit configured to obtain an internal coolant temperature or pressure of the exhaust heat recovery system. Furthermore, the coolant flow rate is restricted at a cold start of the internal combustion engine, and the restriction of the coolant flow rate is released on the basis of the coolant temperature or pressure of the engine coolant passage and the internal coolant temperature or pressure of the exhaust heat recovery system.