A measurement accuracy check system of a sphygmomanometer includes a sphygmomanometer having a blood pressure measurement mode for measuring a blood pressure based on a change in internal pressure of a cuff attached to a blood pressure measurement site, and an accuracy check mode for checking measurement accuracy in the blood pressure measurement mode, and an accuracy check device communicably connected with the sphygmomanometer for determining the measurement accuracy of the sphygmomanometer in the accuracy check mode. The sphygmomanometer includes an air system piping communicating to the cuff in the blood pressure measurement mode and communicating to an air system of the accuracy check device in the accuracy check mode, a pressurization and depressurization unit for adjusting pressure to be applied to the air system piping, and a first pressure detection unit for detecting pressure in the air system piping.