An apparatus for determining the attitude of an artificial satellite by narrowing attitude candidates to a correct one in a short time. The apparatus has star sensors, a star catalog data base, star identification sections in each of which star images output from the star sensors are collated with a star catalog with respect to each star sensor to output a group of corresponding candidates, attitude computation sections for computing a value of an attitude candidate of the artificial satellite with respect to each corresponding candidate, an attitude updating section for updating the value of the attitude candidate on the basis of the star images output from the star sensor and the star catalog, and an attitude propagation section for computing the value of the attitude candidate at the present sampling time from the value of the attitude candidate at the preceding sampling time and the artificial satellite's angular velocity. An attitude candidate unification section for comparing and combining the values of attitude candidates into one candidate is provided in a loop including the attitude updating section and the attitude propagation section.