In Japanese nursing homes, diapers are typically changed at scheduled times each day. However, unnecessary changes may occur when no urination has taken place or when the diaper retains sufficient absorptive capacity. These unnecessary changes increase caregiver workload and contribute to higher costs due to the disposal of unused diapers. Monitoring urination timing and urine absorption volume can help ensure that diapers are changed only when needed, thereby improving care efficiency and reducing costs. We developed a diaper sensor system capable of detecting urination timing and measuring urine absorption volume. The system consists of comb-shaped electrodes connected to a microcomputer, with the electrodes affixed to the outside of the diaper. Urine absorption volume is derived from changes in capacitance measured between the electrodes. The previous system did not consider postures when measuring urine absorption volume. This study demonstrates the necessity of posture detection for measuring urine absorption volume and proposes a new diaper sensor system that measures urine absorption volume using posture detected by an integrated accelerometer. In phantom experiments, the proposed system reduced the mean absolute percentage error (MAPE) from 30% (previous system) to 10%, and posture detection using the accelerometer achieved 100% accuracy, demonstrating the necessity of posture detection for measuring urine absorption volume. To evaluate the practicality of the proposed system and compare its accuracy in measuring urine absorption volume with that of the previous system, a healthy adult male volunteer wore the sensor-attached diaper and accelerometer. After resting for 1 h, the subject urinated in one of three postures-supine, sitting, or 90-degree lateral-each tested eight times. The proposed system reduced the MAPE from 35% (previous) to 23%, demonstrating improved measurement accuracy in actual use scenarios.