A fin-tube type heat exchanger comprising: tubes through which a heat medium flows, the tubes being disposed in parallel at a predetermined distance to allow a combustion product to pass through a space therebetween; and heat transfer fins spaced apart from each other and coupled to outer surfaces of the tubes along a longitudinal direction so that the heat transfer fins are disposed parallel to a flow direction of the combustion product, wherein a first turbulent flow-generating member for generating a turbulent flow in the heat medium is disposed inside each of the tubes, wherein the first turbulent flow-generating member comprises: a flat plate part disposed in the longitudinal direction of the tube to divide an inner space of the tube into two spaces; and first and second guide pieces (132, 133) spaced apart from each other along the longitudinal direction to alternately protrude inclined from both side surfaces of the flat plate part; further comprising plural first guide pieces and plural second guide pieces; and plural first communication holes and plural second communication holes through which the fluid communicates with both spaces of the flat plate part and disposed adjacent to the plural first guide pieces and the plural second guide pieces; characterised in that welding parts (136 137) protrude respectively from front and rear ends of the flat plate part in both directions and are welded and coupled to an inner surface of the tube. tubes through which a heat medium flows, the tubes being disposed in parallel at a predetermined distance to allow a combustion product to pass through a space therebetween; and heat transfer fins spaced apart from each other and coupled to outer surfaces of the tubes along a longitudinal direction so that the heat transfer fins are disposed parallel to a flow direction of the combustion product, wherein a first turbulent flow-generating member for generating a turbulent flow in the heat medium is disposed inside each of the tubes, wherein the first turbulent flow-generating member comprises: a flat plate part disposed in the longitudinal direction of the tube to divide an inner space of the tube into two spaces; and first and second guide pieces (132, 133) spaced apart from each other along the longitudinal direction to alternately protrude inclined from both side surfaces of the flat plate part; further comprising plural first guide pieces and plural second guide pieces; and plural first communication holes and plural second communication holes through which the fluid communicates with both spaces of the flat plate part and disposed adjacent to the plural first guide pieces and the plural second guide pieces; characterised in that welding parts (136 137) protrude respectively from front and rear ends of the flat plate part in both directions and are welded and coupled to an inner surface of the tube. a flat plate part disposed in the longitudinal direction of the tube to divide an inner space of the tube into two spaces; and first and second guide pieces (132, 133) spaced apart from each other along the longitudinal direction to alternately protrude inclined from both side surfaces of the flat plate part; further comprising plural first guide pieces and plural second guide pieces; and plural first communication holes and plural second communication holes through which the fluid communicates with both spaces of the flat plate part and disposed adjacent to the plural first guide pieces and the plural second guide pieces; characterised in that welding parts (136 137) protrude respectively from front and rear ends of the flat plate part in both directions and are welded and coupled to an inner surface of the tube.