A thermal flow meter comprising: a measurement tube made of resin including an inlet through which liquid enters and an outlet through which the liquid flowing from the inlet exits, and having an internal flow passage extending along an axis, wherein the internal flow passage has a circular cross section; a temperature detecting substrate on which a heating resistance element and a temperature detecting resistance element (12b, 12c) configured to detect a temperature of the liquid heated by the heating resistance element are formed on a detection surface thereof along the axis, wherein the temperature detecting resistance element (12b, 12c) is arranged downstream of the heating resistance element; and a reinforcing plate, wherein the detection surface of the temperature detecting substrate is bonded with an adhesive to the measurement tube along the axis, and the reinforcing plate is bonded with the adhesive to the measurement tube along the axis with the internal flow passage interposed between the reinforcing plate and the temperature detecting substrate; characterised in that: the measurement tube has an outer circumferential surface having a rectangular cross section in a plane orthogonal to the axis, the rectangular cross section comprising a first flat surface on which the detection surface of the temperature detecting substrate is oppositely positioned, and a second flat surface on which a face of the reinforcing plate is oppositely positioned with the internal flow passage interposed between the first flat surface and the second flat surface, in that the measurement tube has an outer circumferential surface having a circular cross section in a plane orthogonal to the axis, at points where the temperature detecting substrate and the reinforcing plate are not bonded to the measurement tube, and in that the first flat surface and the detection surface of the temperature detecting substrate are bonded together with the adhesive, and the second flat surface and the face of the reinforcing plate are bonded together with the adhesive. a measurement tube made of resin including an inlet through which liquid enters and an outlet through which the liquid flowing from the inlet exits, and having an internal flow passage extending along an axis, wherein the internal flow passage has a circular cross section; a temperature detecting substrate on which a heating resistance element and a temperature detecting resistance element (12b, 12c) configured to detect a temperature of the liquid heated by the heating resistance element are formed on a detection surface thereof along the axis, wherein the temperature detecting resistance element (12b, 12c) is arranged downstream of the heating resistance element; and a reinforcing plate, wherein the detection surface of the temperature detecting substrate is bonded with an adhesive to the measurement tube along the axis, and the reinforcing plate is bonded with the adhesive to the measurement tube along the axis with the internal flow passage interposed between the reinforcing plate and the temperature detecting substrate; characterised in that: the measurement tube has an outer circumferential surface having a rectangular cross section in a plane orthogonal to the axis, the rectangular cross section comprising a first flat surface on which the detection surface of the temperature detecting substrate is oppositely positioned, and a second flat surface on which a face of the reinforcing plate is oppositely positioned with the internal flow passage interposed between the first flat surface and the second flat surface, in that the measurement tube has an outer circumferential surface having a circular cross section in a plane orthogonal to the axis, at points where the temperature detecting substrate and the reinforcing plate are not bonded to the measurement tube, and in that the first flat surface and the detection surface of the temperature detecting substrate are bonded together with the adhesive, and the second flat surface and the face of the reinforcing plate are bonded together with the adhesive. the measurement tube has an outer circumferential surface having a rectangular cross section in a plane orthogonal to the axis, the rectangular cross section comprising a first flat surface on which the detection surface of the temperature detecting substrate is oppositely positioned, and a second flat surface on which a face of the reinforcing plate is oppositely positioned with the internal flow passage interposed between the first flat surface and the second flat surface, in that the measurement tube has an outer circumferential surface having a circular cross section in a plane orthogonal to the axis, at points where the temperature detecting substrate and the reinforcing plate are not bonded to the measurement tube, and in that the first flat surface and the detection surface of the temperature detecting substrate are bonded together with the adhesive, and the second flat surface and the face of the reinforcing plate are bonded together with the adhesive.