Core structure of heat exchanger

A core structure of a heat exchanger where a core part is arranged between a pair of tanks, the core structure comprising:

• a plurality of flat tubes having an upper wall portion and a lower wall portion;
• and
• a plurality of corrugated inner fins formed in a corrugated shape so that each corrugated inner fin has top portions for being fixed to an inner surface of the upper wall portion, bottom portions for being fixed to an inner surface of the lower wall portion, and slanted portions for connecting the top portion with adjacent bottom portions thereof, the corrugated inner fin having first corrugations located at both end sides of the corrugated inner fin and second corrugations located between the first corrugations, where each of the first and the second corrugations has the top portion and the adjacent slanted portions and project from the bottom portions, wherein
• the first corrugations are set to be smaller in a front-back directional length of the corrugated inner fin than a front-back directional length of the second corrugations.