A diagnosis device for an evaporated fuel processing device arranged to adsorb an evaporated fuel generated in a fuel tank at a refueling by a canister, and to process by introducing the evaporated fuel to an intake system of an internal combustion engine during a driving of the internal combustion engine, the diagnosis device comprising: a pump arranged to pressurize or depressurize a system including the fuel tank and the canister; at least one pressure sensor (36, 37) arranged to sense a pressure within the system; and a fuel temperature sensor arranged to sense a temperature of a fuel within the fuel tank, characterized in that the diagnosis device being configured to judge whether or not a temperature difference between a fuel temperature at a start of driving, and a fuel temperature after an end of the driving is equal to or greater than a predetermined difference, with respect to a request of a leakage diagnosis, to judge that a positive pressure or a negative pressure exists within the fuel tank when the temperature difference between the fuel temperature at the start of driving, and the fuel temperature after the end of the driving is equal to or greater than the predetermined difference, and to select a first leakage diagnosis using the positive pressure or the negative pressure existing within the fuel tank, and to select a second leakage diagnosis using a forcible pressurization or a forcible depressurization by the pump when the temperature difference between the fuel temperature at the start of the driving, and the fuel temperature after the end of the driving is smaller than the predetermined difference, wherein the first leakage diagnosis is prohibited when a relative relationship between an outside air temperature and a fuel temperature at the request of the leakage diagnosis is in a direction in which the temperature difference of the fuel temperature is decreased with time, and wherein a pressure release operation is performed before the operation of the pump when the system is in the positive pressure or the negative pressure which is equal to or greater than a predetermined level when the second leakage diagnosis is selected based on the temperature difference. a pump arranged to pressurize or depressurize a system including the fuel tank and the canister; at least one pressure sensor (36, 37) arranged to sense a pressure within the system; and a fuel temperature sensor arranged to sense a temperature of a fuel within the fuel tank, characterized in that the diagnosis device being configured to judge whether or not a temperature difference between a fuel temperature at a start of driving, and a fuel temperature after an end of the driving is equal to or greater than a predetermined difference, with respect to a request of a leakage diagnosis, to judge that a positive pressure or a negative pressure exists within the fuel tank when the temperature difference between the fuel temperature at the start of driving, and the fuel temperature after the end of the driving is equal to or greater than the predetermined difference, and to select a first leakage diagnosis using the positive pressure or the negative pressure existing within the fuel tank, and to select a second leakage diagnosis using a forcible pressurization or a forcible depressurization by the pump when the temperature difference between the fuel temperature at the start of the driving, and the fuel temperature after the end of the driving is smaller than the predetermined difference, wherein the first leakage diagnosis is prohibited when a relative relationship between an outside air temperature and a fuel temperature at the request of the leakage diagnosis is in a direction in which the temperature difference of the fuel temperature is decreased with time, and wherein a pressure release operation is performed before the operation of the pump when the system is in the positive pressure or the negative pressure which is equal to or greater than a predetermined level when the second leakage diagnosis is selected based on the temperature difference.