A nucleic acid sequence measuring apparatus for measuring a target having a specific nucleic acid sequence included in a sample, the nucleic acid sequence measuring apparatus comprising: a detector configured to detect fluorescence emitted from a nucleic acid sequence measuring device which emits fluorescence due to an addition of the target; and a calculator configured to measure the target based on a difference between a first amount of light indicating an amount of fluorescent light emitted from a predefined measurement region of the nucleic acid sequence measuring device at a first time point before or immediately after an addition of the sample to the nucleic acid sequence measuring device and a second amount of light indicating an amount of fluorescent light emitted from the measurement region at a second time point after a predefined time has elapsed from the addition of the sample to the nucleic acid sequence measuring device, based on a detection result of the detector, wherein the detector comprises an image acquirer configured to acquire an image of an image acquisition region including at least the measurement region, and wherein the calculator is configured to perform image processing on a first image that is an image of the image acquisition region acquired at the first time point and a second image that is an image of the image acquisition region acquired at the second time point to acquire the first amount of light and the second amount of light, characterized in that the calculator is configured to perform, as the image processing, a process of acquiring, as the first amount of light, an average gradation value of pixels forming an image of the measurement region included in the first image and a process of acquiring, as the second amount of light, an average gradation value of pixels forming an image of the measurement region included in the second image, in that the calculator is configured to extract a pixel in which a difference in gradation value for each of the pixels between the first image and the second image exceeds a predefined first threshold value, and in that the calculator is configured to perform a process of acquiring, as the first amount of light, an average gradation value of the extracted pixels of the first image, and a process of acquiring, as the second amount of light, an average gradation value of the extracted pixels of the second image. a detector configured to detect fluorescence emitted from a nucleic acid sequence measuring device which emits fluorescence due to an addition of the target; and a calculator configured to measure the target based on a difference between a first amount of light indicating an amount of fluorescent light emitted from a predefined measurement region of the nucleic acid sequence measuring device at a first time point before or immediately after an addition of the sample to the nucleic acid sequence measuring device and a second amount of light indicating an amount of fluorescent light emitted from the measurement region at a second time point after a predefined time has elapsed from the addition of the sample to the nucleic acid sequence measuring device, based on a detection result of the detector, wherein the detector comprises an image acquirer configured to acquire an image of an image acquisition region including at least the measurement region, and wherein the calculator is configured to perform image processing on a first image that is an image of the image acquisition region acquired at the first time point and a second image that is an image of the image acquisition region acquired at the second time point to acquire the first amount of light and the second amount of light, characterized in that the calculator is configured to perform, as the image processing, a process of acquiring, as the first amount of light, an average gradation value of pixels forming an image of the measurement region included in the first image and a process of acquiring, as the second amount of light, an average gradation value of pixels forming an image of the measurement region included in the second image, in that the calculator is configured to extract a pixel in which a difference in gradation value for each of the pixels between the first image and the second image exceeds a predefined first threshold value, and in that the calculator is configured to perform a process of acquiring, as the first amount of light, an average gradation value of the extracted pixels of the first image, and a process of acquiring, as the second amount of light, an average gradation value of the extracted pixels of the second image.