Heat exchanger has a first surface on which a first fluid flows and a second surface on which a second fluid flows, at least one of the first and second surfaces has at least two projections and a bottom therebetween, and an imaginary line on the bottom along a shortest distance between the projections adjacent to each other is prevented from extending along an imaginary face substantially parallel to a main flow direction of one of the first and second fluids which flows over the bottom.
