1. A radiographic image capturing apparatus comprising: an auxiliary image capturing unit configured to continuously capture a plurality of images of a subject with small doses of radiation; a storage unit configured to store the images captured by the auxiliary image capturing unit as a plurality of auxiliary images; a detection unit configured to detect periodicity of motion of the subject based on the auxiliary images; a generation unit configured to generate a pseudo image from those auxiliary images exhibiting the detected periodicity; a calculation unit configured to analyze the generated pseudo image and, based on an analysis result, calculate an image capturing parameter used for main image capturing of a still image of the subject; and a main image capturing unit configured to perform the main image capturing of the still image of the subject using the image capturing parameter.