A method for quantifying a target nucleic acid sequence in a nucleic acid sample using a common internal control without the use of a standard curve, comprising: (a) preparing (i) a control reaction mixture comprising a known-amount standard and an internal control, and (ii) a sample reaction mixture comprising an internal control identical to the internal control in the control reaction mixture and the target nucleic acid sequence to be quantified in separated vessels respectively, whereby the control reaction mixture and the sample reaction mixture each comprises the common internal control; (b) performing an amplification reaction by using the control reaction mixture and the sample reaction mixture; wherein the control reaction mixture is subject to the amplification reaction by using a primer for the standard and a primer for the internal control; wherein the sample reaction mixture is subject to the amplification reaction by using a primer for the target nucleic acid sequence and a primer for the internal control; wherein the primer for the standard and the primer for the internal control are different from each other; and wherein the primer for the target nucleic acid sequence and the primer for the internal control are different from each other; (c) obtaining values reflecting amounts of amplified products of the standard and the internal control in the control reaction mixture, and values reflecting amounts of amplified products of the internal control and the target nucleic acid sequence in the sample reaction mixture, wherein the values are obtained by monitoring the amplification reaction or by analysis after the amplification reaction; (d) calculating a relative ratio of the amount of the standard and the amount of the target nucleic acid sequence by using values reflecting amounts of amplified products of the standard and the internal control in the control reaction mixture, and values reflecting amounts of amplified products of the internal control and the target nucleic acid sequence in the sample reaction mixture; and (e) determining an initial amount of the target nucleic acid sequence in the nucleic acid sample by using a known amount of the standard and the relative ratio calculated in the step (d). (a) preparing (i) a control reaction mixture comprising a known-amount standard and an internal control, and (ii) a sample reaction mixture comprising an internal control identical to the internal control in the control reaction mixture and the target nucleic acid sequence to be quantified in separated vessels respectively, whereby the control reaction mixture and the sample reaction mixture each comprises the common internal control; (b) performing an amplification reaction by using the control reaction mixture and the sample reaction mixture; wherein the control reaction mixture is subject to the amplification reaction by using a primer for the standard and a primer for the internal control; wherein the sample reaction mixture is subject to the amplification reaction by using a primer for the target nucleic acid sequence and a primer for the internal control; wherein the primer for the standard and the primer for the internal control are different from each other; and wherein the primer for the target nucleic acid sequence and the primer for the internal control are different from each other; (c) obtaining values reflecting amounts of amplified products of the standard and the internal control in the control reaction mixture, and values reflecting amounts of amplified products of the internal control and the target nucleic acid sequence in the sample reaction mixture, wherein the values are obtained by monitoring the amplification reaction or by analysis after the amplification reaction; (d) calculating a relative ratio of the amount of the standard and the amount of the target nucleic acid sequence by using values reflecting amounts of amplified products of the standard and the internal control in the control reaction mixture, and values reflecting amounts of amplified products of the internal control and the target nucleic acid sequence in the sample reaction mixture; and (e) determining an initial amount of the target nucleic acid sequence in the nucleic acid sample by using a known amount of the standard and the relative ratio calculated in the step (d).