A fuel property estimation device for an internal combustion engine that uses a mixed fuel of three kinds of fuel, including a first sensor provided on a fuel route and a second sensor provided on an exhaust passage, the first sensor outputting a signal responsive to a physical property of the fuel, and a second sensor outputting a signal responsive to an oxygen concentration of exhaust gas, the fuel property estimation device characterized by comprising: physical property measurement means for measuring a physical property value of the mixed fuel used in the internal combustion engine based on a signal from the first sensor; air-fuel ratio value calculation means for calculating an air-fuel ratio value at a time when the combustion state of the internal combustion engine is adjusted to be stoichiometric using feedback of a signal from the second sensor; and fuel composition ratio estimation means for estimating a composition ratio of the mixed fuel used in the internal combustion engine based on the physical property value measured by the physical property value measurement means and the air-fuel ratio value calculated by the air-fuel ratio value calculation means by referring to a relationship between the composition ratio of the mixed fuel and the physical property value and a relationship between the composition ratio of the mixed fuel and a theoretical air-fuel ratio value, the fuel composition ratio estimation means comprising: storage means for storing a first table that prescribes the relationship between the composition ratio of the mixed fuel and the physical property value and a second table that prescribes the relationship between the composition ratio of the mixed fuel and the theoretical air-fuel ratio value; means for selecting a first group of candidates of the composition ratio of the mixed fuel by referring to the first table for the physical property value measured by the physical property value measurement means; means for selecting a second group of candidates of the composition ratio of the mixed fuel by referring to the second table for the air-fuel ratio value calculated by the air-fuel ratio value calculation means; means for searching for a common candidate between the first group of candidates and the second group of candidates by comparing the first group of candidates and the second group of candidates; and means for estimating the common candidate as the composition ratio of the mixed fuel used in the internal combustion engine. physical property measurement means for measuring a physical property value of the mixed fuel used in the internal combustion engine based on a signal from the first sensor; air-fuel ratio value calculation means for calculating an air-fuel ratio value at a time when the combustion state of the internal combustion engine is adjusted to be stoichiometric using feedback of a signal from the second sensor; and fuel composition ratio estimation means for estimating a composition ratio of the mixed fuel used in the internal combustion engine based on the physical property value measured by the physical property value measurement means and the air-fuel ratio value calculated by the air-fuel ratio value calculation means by referring to a relationship between the composition ratio of the mixed fuel and the physical property value and a relationship between the composition ratio of the mixed fuel and a theoretical air-fuel ratio value, the fuel composition ratio estimation means comprising: storage means for storing a first table that prescribes the relationship between the composition ratio of the mixed fuel and the physical property value and a second table that prescribes the relationship between the composition ratio of the mixed fuel and the theoretical air-fuel ratio value; means for selecting a first group of candidates of the composition ratio of the mixed fuel by referring to the first table for the physical property value measured by the physical property value measurement means; means for selecting a second group of candidates of the composition ratio of the mixed fuel by referring to the second table for the air-fuel ratio value calculated by the air-fuel ratio value calculation means; means for searching for a common candidate between the first group of candidates and the second group of candidates by comparing the first group of candidates and the second group of candidates; and means for estimating the common candidate as the composition ratio of the mixed fuel used in the internal combustion engine. storage means for storing a first table that prescribes the relationship between the composition ratio of the mixed fuel and the physical property value and a second table that prescribes the relationship between the composition ratio of the mixed fuel and the theoretical air-fuel ratio value; means for selecting a first group of candidates of the composition ratio of the mixed fuel by referring to the first table for the physical property value measured by the physical property value measurement means; means for selecting a second group of candidates of the composition ratio of the mixed fuel by referring to the second table for the air-fuel ratio value calculated by the air-fuel ratio value calculation means; means for searching for a common candidate between the first group of candidates and the second group of candidates by comparing the first group of candidates and the second group of candidates; and means for estimating the common candidate as the composition ratio of the mixed fuel used in the internal combustion engine.