A cooling system includes: a first coolant passage; a second coolant passage; a pump; a radiator; a third coolant passage; a connection switching mechanism that switches between a forward flow connection state and a reverse flow connection state; a fourth coolant passage; a fifth coolant passage; and a shutoff valve configured to open/shut off the fifth coolant passage. The radiator is disposed at a location at which coolant flowing from a second end of the first coolant passage into a fourth end of the second coolant passage is not cooled in the reverse flow connection state, and coolant flowing out from the second end of the first coolant passage and the fourth end of the second coolant passage is cooled in the forward flow connection state.