The heat exchanger has a zig-zagged liquid-flowing passage and an overflowed-liquid escaping conduit. When an excessive amount of cooling water is supplied to the heat exchanger, a part of cooling water flows into the overflowed-liquid escaping conduit through a dam formed between the liquid-flowing passage and the escaping conduit. The heat exchanger is formed by assembling a plurality of heat exchanging partition plates formed by molding, which have the same shape and the same dimensions, and which are easily engageable with and detachable form each other.