An economizer for warming water by combustion exhaust gas generated by a boiler, comprising: a cylindrical water pipe in which an inflow port and an outflow port are formed on a side surface and through which the water passes; a combustion exhaust gas introduction pipe connected to a lower end position of the water pipe via a partition wall; a combustion exhaust gas discharge pipe connected to an upper end position of the water pipe via a partition wall; wherein the inside of the combustion exhaust gas introduction pipe is partitioned into a combustion exhaust gas introduction chamber facing a gas introduction port and a lower combustion exhaust gas passage chamber, and the inside of the combustion exhaust gas discharge pipe is partitioned into a combustion exhaust gas discharge chamber facing a gas exhaust port and an upper combustion exhaust gas passage chamber, a plurality of first gas pipes erected in the water pipe, penetrating the partition walls (14, 17) so as to communicate the combustion exhaust gas introduction chamber and the upper combustion exhaust gas passage chamber; a plurality of second gas pipes erected in the water pipe, penetrating the partition walls (14, 17) so as to communicate the upper combustion exhaust gas passage chamber and the lower combustion exhaust gas passage chamber; and a plurality of third gas pipes erected in the water pipe, penetrating the partition walls (14, 17) so as to communicate the lower combustion exhaust gas passage chamber and the combustion exhaust gas discharge chamber, wherein the combustion exhaust gas introduction chamber is formed with an area where the combustion exhaust gas introduction pipe is divided into three equal parts in a horizontal plane, and the combustion exhaust gas discharge chamber is formed with an area where the combustion exhaust gas discharge pipe is divided into three equal parts in a horizontal plane, so that passages where the first gas pipes, the second gas pipes, and the third gas pipes are erected each have the same area, whereas a bottom surface lid detachably attached to a position excluding the gas introduction port provided on a lower surface side of the combustion exhaust gas introduction pipe, and an upper surface lid detachably attached to a position excluding the gas exhaust port provided on an upper surface side of the combustion exhaust gas discharge pipe are provided, and both ends of the first gas pipes, the second gas pipes, and the third gas pipes can be inspected in a state in which the bottom surface lid and the upper surface lid are removed. a cylindrical water pipe in which an inflow port and an outflow port are formed on a side surface and through which the water passes; a combustion exhaust gas introduction pipe connected to a lower end position of the water pipe via a partition wall; a combustion exhaust gas discharge pipe connected to an upper end position of the water pipe via a partition wall; wherein the inside of the combustion exhaust gas introduction pipe is partitioned into a combustion exhaust gas introduction chamber facing a gas introduction port and a lower combustion exhaust gas passage chamber, and the inside of the combustion exhaust gas discharge pipe is partitioned into a combustion exhaust gas discharge chamber facing a gas exhaust port and an upper combustion exhaust gas passage chamber, a plurality of first gas pipes erected in the water pipe, penetrating the partition walls (14, 17) so as to communicate the combustion exhaust gas introduction chamber and the upper combustion exhaust gas passage chamber; a plurality of second gas pipes erected in the water pipe, penetrating the partition walls (14, 17) so as to communicate the upper combustion exhaust gas passage chamber and the lower combustion exhaust gas passage chamber; and a plurality of third gas pipes erected in the water pipe, penetrating the partition walls (14, 17) so as to communicate the lower combustion exhaust gas passage chamber and the combustion exhaust gas discharge chamber, wherein the combustion exhaust gas introduction chamber is formed with an area where the combustion exhaust gas introduction pipe is divided into three equal parts in a horizontal plane, and the combustion exhaust gas discharge chamber is formed with an area where the combustion exhaust gas discharge pipe is divided into three equal parts in a horizontal plane, so that passages where the first gas pipes, the second gas pipes, and the third gas pipes are erected each have the same area, whereas a bottom surface lid detachably attached to a position excluding the gas introduction port provided on a lower surface side of the combustion exhaust gas introduction pipe, and an upper surface lid detachably attached to a position excluding the gas exhaust port provided on an upper surface side of the combustion exhaust gas discharge pipe are provided, and both ends of the first gas pipes, the second gas pipes, and the third gas pipes can be inspected in a state in which the bottom surface lid and the upper surface lid are removed.