The present invention provides an amplification determining method that can determine whether or not an objective nucleic acid has been amplified with respect to a sample treated so as to amplify the nucleic acid. Signal values showing molten states of the treated sample at respective temperatures are provided, and the maximum signal value (A) is searched for. Further, signal differential values at the respective temperatures are calculated by differentiation of the signal values at two successive points, and second-order differential values of the differential values are calculated by differentiation at four successive points. Among the second differential values, from those in a predetermined temperature range including a Tm value of the objective nucleic acid, the maximum second differential value (Dmax′) and the minimum second differential value (Dmin′) are selected. Then, the maximum difference (B) is calculated based on the formula (B)=(Dmax′)−(Dmin′). The calculation of the formula X=(B)/(A) is performed, and when X satisfies the condition [X>predetermined threshold value], the amplification is determined as normal amplification, and when X satisfies the condition [X≦predetermined threshold value], the amplification is determined as poor amplification.