A heat exchanger efficiently and uniformly cools or heats portions to be controlled to a prescribed temperature, and then provides a surface processing apparatus which makes it possible to continuously carry out stable processing. The heat exchanger is constructed by arranging partition walls between two plates to form a fluid channel and a fin parallel with the channel or inclined by a prescribed angle on each of the two plates insides the channel so that the plate or a member in contact with the plate is cooled or heated with the fluid flowing through the channel.