A fin tube heat exchanger comprising: a plurality of fins arranged in parallel for forming a gas passage; and a heat transfer tube penetrating the plurality of fins, the heat transfer tube being configured to allow a medium that exchanges heat with the gas to flow through the heat transfer tube, wherein each of the fins is a corrugated fin shaped such that a peak portion appears only at one location in an air stream direction, the fins each comprising: a plurality of through holes into which the heat transfer tube is fitted; a flat portion formed around the through hole; a first inclined portion being inclined relative to the air stream direction so as to form the peak portion; and a second inclined portion connecting the flat portion and the first inclined portion, the plurality of through holes are formed along a step direction perpendicular to both a direction in which the plurality of fins are arranged and the air stream direction, characterised in that when a distance from an upstream end to a downstream end of the first inclined portion in the air stream direction is defined as SI, a distance from an upstream end to a downstream end of the flat portion in the air stream direction is defined as D1, a plane contacting the upstream end and the downstream end of the first inclined portion in the air stream direction from a side opposite to an apex side of the peak portion is defined as a reference plane, an angle formed between the reference plane and the first inclined portion is defined as θ1, an angle formed between the reference plane and the second inclined portion in an area on an upstream side in the air stream direction as viewed from the through hole is defined as θ2, a distance from the reference plane to the flat portion is defined as α, and a distance between the reference plane of one of the fins and the reference plane of another of the fins adjacent to the apex side of the peak portion is defined as L, in a case where the flat portion is on a side same as the apex side of the peak portion with respect to the reference plane, or in a case of α = 0, the following relationship holds true: θ° < θ 2 < tan − 1 L − α / S 1 − D 1 / 2 − L / tanθ 1, and in a case where the flat portion is on a side opposite to the apex side of the peak portion with respect to the reference plane, the following relationship holds true: θ° < θ 2 < tan − 1 L + α / S 1 − D 1 / 2 − L / tanθ 1. a plurality of fins arranged in parallel for forming a gas passage; and a heat transfer tube penetrating the plurality of fins, the heat transfer tube being configured to allow a medium that exchanges heat with the gas to flow through the heat transfer tube, wherein each of the fins is a corrugated fin shaped such that a peak portion appears only at one location in an air stream direction, the fins each comprising: a plurality of through holes into which the heat transfer tube is fitted; a flat portion formed around the through hole; a first inclined portion being inclined relative to the air stream direction so as to form the peak portion; and a second inclined portion connecting the flat portion and the first inclined portion, the plurality of through holes are formed along a step direction perpendicular to both a direction in which the plurality of fins are arranged and the air stream direction,