PURPOSE: To provide a compact, light weight and low priced flowing liquid film type vaporizer by disposing a level which utilizes pipes having axially arranged grooves as heat transmission pipes at a suitable location of a pipe stack.

CONSTITUTION: Multiple heat transmissions pipes 2 are arranged in a shell 1 to form multi-level, horizontal pipe stacks 3. A liquid distribution mechanism 4 is disposed above the pipe stacks 3. The liquid A' of to-be-vaporized fluid is first supplied to said liquid distribution mechanism 4, and then, uniformly flows down to the pipe stacks 3 below as it forms a thin liquid film on the outer surface of each heat transmission pipe 2. During the process of flowing down, it is heated by heating fluid B flowing in the heat transmission pipes to be vaporized to become vapor A" which is discharged outside the shell 1. Most of the heat transmission pipes that make up the pipe stacks are low finned pipes or nuclear boiling precipitating pipes. However, there is a level at a suitable location in which pipes 5 having axially arranged grooves are employed. Since the axially grooved pipes have an effect to equalize the liquid in the lateral direction, the uneven liquid film can be corrected by disposing such pipes at a suitable location, allowing to form a desirable liquid film on all of the heat transmission pipes.