An abnormality detection device for an internal combustion engine whose operation is controlled in accordance with properties of an employed fuel, the abnormality detection device comprising: a branch flow path (40; 26; 38) branched off from a primary flow path (20, 24, 30, 32, 34) of a fuel flow path connecting a fuel pump to an injector, wherein the fuel flows to the branch flow path (40; 26; 38) from the primary flow path (20, 24, 30, 32, 34) when the fuel pump is operating and the fuel in the branch flow path (40; 26; 38) flows out to empty the branch flow path (40; 26; 38) when the fuel pump stops; a fuel property sensor (52; 54; 56) disposed in the branch flow path (40; 26; 38), the fuel property sensor (52; 54; 56) generating different output values depending on whether a liquid or a gas exists in a measurement section, and when a fuel exists in the measurement section, the output value is determined in accordance with the properties of the fuel; first sensor output value acquisition means for acquiring a first sensor output value of the fuel property sensor (52; 54; 56), the first sensor output value being an output value generated by the fuel property sensor (52; 54; 56) in a situation where the fuel is flowing through the branch flow path (40; 26; 38); second sensor output value acquisition means for acquiring a second sensor output value of the fuel property sensor (52; 54; 56), the second sensor output value being an output value generated by the fuel property sensor (52; 54; 56) in a situation where the fuel in the branch flow path (40; 26; 38) has flown out; and abnormality judgment means for checking for an abnormality in the fuel property sensor (52; 54; 56) by using the first sensor output value and the second sensor output value as abnormality judgment data. a branch flow path (40; 26; 38) branched off from a primary flow path (20, 24, 30, 32, 34) of a fuel flow path connecting a fuel pump to an injector, wherein the fuel flows to the branch flow path (40; 26; 38) from the primary flow path (20, 24, 30, 32, 34) when the fuel pump is operating and the fuel in the branch flow path (40; 26; 38) flows out to empty the branch flow path (40; 26; 38) when the fuel pump stops; a fuel property sensor (52; 54; 56) disposed in the branch flow path (40; 26; 38), the fuel property sensor (52; 54; 56) generating different output values depending on whether a liquid or a gas exists in a measurement section, and when a fuel exists in the measurement section, the output value is determined in accordance with the properties of the fuel; first sensor output value acquisition means for acquiring a first sensor output value of the fuel property sensor (52; 54; 56), the first sensor output value being an output value generated by the fuel property sensor (52; 54; 56) in a situation where the fuel is flowing through the branch flow path (40; 26; 38); second sensor output value acquisition means for acquiring a second sensor output value of the fuel property sensor (52; 54; 56), the second sensor output value being an output value generated by the fuel property sensor (52; 54; 56) in a situation where the fuel in the branch flow path (40; 26; 38) has flown out; and abnormality judgment means for checking for an abnormality in the fuel property sensor (52; 54; 56) by using the first sensor output value and the second sensor output value as abnormality judgment data.