A sphygmomanometer having a built-in arrhythmia detecting mechanism, wherein the blood pressure measuring portion contains a pulsation detector, which receives pulse sounds including Korotkoff sounds for electrically converting the pulse sounds into wave form signals of the vascular pulse wave and output the same, and a mechanism indicating the blood pressure value by means of detecting the Korotkoff sounds recognized and separated from the wave form signals of the vascular pulse wave output in the above-mentioned way; the incorporated arrhythmia detecting mechanism receives the wave form signals of the vascular pulse wave for comparing the cycle of the wave form signals with a predetermined criterion in order to electrically detect irregularities, if any, in the signals for detecting the existence of arrhythmias. This sphygmomanometer is capable of measuring the blood pressure simultaneously with the detecting of arrhythmia.