A fin tube heat exchanger comprising: a plurality of fins arranged parallel to each other to define passages of a gaseous fluid; and a heat transfer tube extending through the plurality of fins and allowing a medium that exchanges heat with the gaseous fluid to flow therethrough, wherein each of the plurality of fins is a corrugated fin that has only one peak portion when viewed in an air flow direction, the plurality of fins each includes: a plurality of through holes to which the heat transfer tube is fitted; a cylindrical fin collar disposed to extend around each of the through holes while being in close contact with the heat transfer tube; a flat portion extending around the fin collar; a first inclined portion inclined with respect to the air flow 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 arranged in a row direction which is perpendicular to both an arrangement direction of the plurality of fins and the air flow direction, the plurality of fins each satisfy a relation below if the flat portion is positioned closer than a reference plane to a top of the peak portion or is positioned to satisfy α = 0, in which the reference plane is a plane in contact with a surface of each of an upstream end and a downstream end in the air flow direction of the first inclined portion that is opposite a surface thereof adjacent to the top of the peak portion and α is a distance between the reference plane and the flat portion, 0 ° ≤ θ 2 < tan − 1 L − α / S 1 − D 1 / 2 − L / tan θ 1 where S1 is a distance between the upstream end and the downstream end of the first inclined portion in the air flow direction, D1 is a distance between an upstream end and a downstream end of the flat portion in the air flow direction, θ1 is an angle between the reference plane and the first inclined portion in the air flow direction, θ2 is an angle between the reference plane and the second inclined portion in the air flow direction, and L is a distance between the reference plane of one of the plurality of fins and the reference plane of an adjacent one of the plurality of fins adjacent to the top of the peak portion, and the plurality of fins each satisfy a relation below if the flat portion is positioned further than the reference plane from the top of the peak portion, 0 ° < θ 2 < tan − 1 L + α / S 1 − D 1 / 2 − L / tan θ 1 characterized in that the angle between the reference plane and the second inclined portion gradually decreases as a measurement direction of the angle is shifted from the row direction to the air flow direction and is minimum when the measurement direction is oriented in the air flow direction. a plurality of fins arranged parallel to each other to define passages of a gaseous fluid; and a heat transfer tube extending through the plurality of fins and allowing a medium that exchanges heat with the gaseous fluid to flow therethrough, wherein each of the plurality of fins is a corrugated fin that has only one peak portion when viewed in an air flow direction, the plurality of fins each includes: a plurality of through holes to which the heat transfer tube is fitted; a plurality of through holes to which the heat transfer tube is fitted; a cylindrical fin collar disposed to extend around each of the through holes while being in close contact with the heat transfer tube; a flat portion extending around the fin collar; a first inclined portion inclined with respect to the air flow 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 arranged in a row direction which is perpendicular to both an arrangement direction of the plurality of fins and the air flow direction, the plurality of fins each satisfy a relation below if the flat portion is positioned closer than a reference plane to a top of the peak portion or is positioned to satisfy α = 0, in which the reference plane is a plane in contact with a surface of each of an upstream end and a downstream end in the air flow direction of the first inclined portion that is opposite a surface thereof adjacent to the top of the peak portion and α is a distance between the reference plane and the flat portion, 0 ° ≤ θ 2 < tan − 1 L − α / S 1 − D 1 / 2 − L / tan θ 1 where S1 is a distance between the upstream end and the downstream end of the first inclined portion in the air flow direction, D1 is a distance between an upstream end and a downstream end of the flat portion in the air flow direction, θ1 is an angle between the reference plane and the first inclined portion in the air flow direction, θ2 is an angle between the reference plane and the second inclined portion in the air flow direction, and L is a distance between the reference plane of one of the plurality of fins and the reference plane of an adjacent one of the plurality of fins adjacent to the top of the peak portion, and the plurality of fins each satisfy a relation below if the flat portion is positioned further than the reference plane from the top of the peak portion, 0 ° < θ 2 < tan − 1 L + α / S 1 − D 1 / 2 − L / tan θ 1 characterized in that the angle between the reference plane and the second inclined portion gradually decreases as a measurement direction of the angle is shifted from the row direction to the air flow direction and is minimum when the measurement direction is oriented in the air flow direction.