An unequal-interval combustion engine misfire-determination device determines whether a misfire has occurred in an unequal-interval combustion engine including a plurality of cylinders in which combustion occurs at unequal intervals. The misfire determination device includes: an unequal interval combustion fluctuation canceled value acquisition unit that acquires, based on a crank angle signal, an unequal interval combustion fluctuation canceled value which is a rotation speed in a section of 720×m crank angle degrees including a predetermined determination angle position, where m is a natural number; an unequal interval combustion fluctuation manifested value calculation unit that calculates an unequal interval combustion fluctuation manifested value in which a fluctuation component attributable to unequal interval combustion is manifested, by removing the unequal interval combustion fluctuation canceled value; and a specific cylinder misfire determination unit that determines a misfire in a specific cylinder based on the unequal interval combustion fluctuation manifested value.