It is well known that taking wrong sitting posture all day long is harmful for health. However, quantifying the degree of collapse of posture is not so easy. Typically, checking the video of sitting state or examining the spinal curves in radiograph is made so far, but it needs a clinical specialist for diagnosis. In this paper, for attaining this goal more easily, we give a device usable in daily life. We measure a time series of the pressure distributions obtained from a mat with 16×16 pressure sensors put on a chair. More concretely we analyze the sitting person's move direction and the amount, and make a daily diagnosis report aiming at helping his/her own notice. Through the experiments using two subjects who spent around three hours at the chair, a possibility of automatic and daily-basis diagnosis of sitting posture is demonstrated.