A fin-tube heat exchanger comprising: a stack of heat transfer fins (3A, 3B); and a heat transfer tube penetrating the stack of heat transfer fins (3A, 3B), wherein each of the heat transfer fins (3A, 3B) comprises: a base portion; a cylindrical collar portion extending upwardly from the base portion; a protruding portion protruding radially outwardly from a part of an upper end of the collar portion; a wall portion extending upwardly from a part of the upper end of the collar portion other than the part of the upper end from which the protruding portion protrudes, a bottom portion provided on a lower end of the collar portion, the bottom portion being curved and extending radially outwardly from a bottom of the collar portion; wherein a height B from the base portion to a top of the wall portion is greater than a height A from the base portion to a top of the protruding portion, a notched portion is provided between the protruding portion and the wall portion, and characterized in that the heat transfer fins further comprise a stepped portion that is curved and extends from the bottom portion down to the base portion to form a recess, wherein the protruding portion of one of the heat transfer fins (3A, 3B) in the stack enters the recess formed by the stepped portion of another heat transfer fin (3A, 3B) that is stacked on the one heat transfer fin (3A, 3B). a stack of heat transfer fins (3A, 3B); and a heat transfer tube penetrating the stack of heat transfer fins (3A, 3B), wherein each of the heat transfer fins (3A, 3B) comprises: a base portion; a cylindrical collar portion extending upwardly from the base portion; a protruding portion protruding radially outwardly from a part of an upper end of the collar portion; a wall portion extending upwardly from a part of the upper end of the collar portion other than the part of the upper end from which the protruding portion protrudes, a bottom portion provided on a lower end of the collar portion, the bottom portion being curved and extending radially outwardly from a bottom of the collar portion; wherein a height B from the base portion to a top of the wall portion is greater than a height A from the base portion to a top of the protruding portion, a notched portion is provided between the protruding portion and the wall portion, and characterized in that the heat transfer fins further comprise a stepped portion that is curved and extends from the bottom portion down to the base portion to form a recess, wherein the protruding portion of one of the heat transfer fins (3A, 3B) in the stack enters the recess formed by the stepped portion of another heat transfer fin (3A, 3B) that is stacked on the one heat transfer fin (3A, 3B). a base portion; a cylindrical collar portion extending upwardly from the base portion; a protruding portion protruding radially outwardly from a part of an upper end of the collar portion; a wall portion extending upwardly from a part of the upper end of the collar portion other than the part of the upper end from which the protruding portion protrudes, a bottom portion provided on a lower end of the collar portion, the bottom portion being curved and extending radially outwardly from a bottom of the collar portion; wherein a height B from the base portion to a top of the wall portion is greater than a height A from the base portion to a top of the protruding portion, a notched portion is provided between the protruding portion and the wall portion, and characterized in that the heat transfer fins further comprise a stepped portion that is curved and extends from the bottom portion down to the base portion to form a recess, wherein the protruding portion of one of the heat transfer fins (3A, 3B) in the stack enters the recess formed by the stepped portion of another heat transfer fin (3A, 3B) that is stacked on the one heat transfer fin (3A, 3B).