A treatment device for a medical treatment which heats a biotissue at a desired temperature to perform the medical treatment, the device comprising: a treatment tool which includes a heat transfer portion (132, 134) which comes in contact with the biotissue to transfer heat to the biotissue, and a resistance element into which power is supplied to heat the heat transfer portion (132, 134); and a controller which is detachably attached to the treatment tool, and configured to measure a resistance value R of the resistance element, and supplies the power to the resistance element so as to control a temperature of the heat transfer portion (132, 134) to heat the biotissue at the desired temperature, characterized in that the device further comprises a storage section (123; 187) storing a coefficient C2, and the controller is configured to calculate a coefficient C1 before the medical treatment while the treatment tool and the controller are connected to each other, based on a calibration temperature Tc, the stored coefficient C2 and a measured resistance value Rc of the resistance element, in accordance with C1 =(Tc-C2)/Rc, calculate a temperature T of the resistance element, using the coefficient C1, the resistance value R and the coefficient C2, in accordance with T = C1 x R + C2, and control the temperature of the heat transfer portion (132, 134) by use of the temperature T. a treatment tool which includes a heat transfer portion (132, 134) which comes in contact with the biotissue to transfer heat to the biotissue, and a resistance element into which power is supplied to heat the heat transfer portion (132, 134); and a heat transfer portion (132, 134) which comes in contact with the biotissue to transfer heat to the biotissue, and a resistance element into which power is supplied to heat the heat transfer portion (132, 134); and a controller which is detachably attached to the treatment tool, and configured to measure a resistance value R of the resistance element, and supplies the power to the resistance element so as to control a temperature of the heat transfer portion (132, 134) to heat the biotissue at the desired temperature, characterized in that the device further comprises a storage section (123; 187) storing a coefficient C2, and the controller is configured to calculate a coefficient C1 before the medical treatment while the treatment tool and the controller are connected to each other, based on a calibration temperature Tc, the stored coefficient C2 and a measured resistance value Rc of the resistance element, in accordance with C1 =(Tc-C2)/Rc, calculate a temperature T of the resistance element, using the coefficient C1, the resistance value R and the coefficient C2, in accordance with T = C1 x R + C2, and control the temperature of the heat transfer portion (132, 134) by use of the temperature T. calculate a coefficient C1 before the medical treatment while the treatment tool and the controller are connected to each other, based on a calibration temperature Tc, the stored coefficient C2 and a measured resistance value Rc of the resistance element, in accordance with C1 =(Tc-C2)/Rc, calculate a temperature T of the resistance element, using the coefficient C1, the resistance value R and the coefficient C2, in accordance with T = C1 x R + C2, and control the temperature of the heat transfer portion (132, 134) by use of the temperature T.