A guide plate having depressed portions is provided between an array of heat exchanger tubes, herein after “tubes”, arranged horizontally side-by-side and a next lower array of tubes arranged horizontally side-by-side, and is positioned with the lowest parts of the depressed portions near crest portions of respective lower tubes. The guide plate conveys a liquid D on outer surfaces of respective upper tubes onto similarly positioned lower tubes even when the tubes move in a right-and-left direction. A falling film heat exchanger installed in a ship, an offshore structure or the like can avoid reduction in heat exchange performance, even when the ship or the like inclines and swings, by substantially evenly distributing and dropping a liquid onto the crests of the tubes and causing the liquid dropped from the tubes located in an upper array to fall onto the tubes located in the next lower array.