This paper proposes a novel technique to estimate continuous pulse pressure waves based on aortic pulse waves (APWs) that are measured in an unconstrained manner at the back of a user via an APW sensor. Frequency filtering and rectification are applied to the APWs as a preprocessing step. These preprocessed waves are then converted into continuous pulse pressure waves using a black box model that includes the physical factors of the patient. The black box model is constructed beforehand using the preprocessed APWs and continuous arterial pressure waves measured from other users. The converted pulse pressure waves were then compared with continuous arterial pressure waves measured using a commercial sphygmomanometer to evaluate the accuracy of the proposed method. As a result, the correlation coefficient between the converted waves from the proposed method and the waves measured using the sphygmomanometer was 0.83 or higher and the mean absolute error was 6.07 ± 5.89 mmHg. It was therefore concluded that the proposed method may enable unconstrained measurement of continuous pulse pressure waves.