Abstract: An apparatus which can measure the blood pressure automatically, continuously and indirectly, is provided. The apparatus is implemented basing upon the theory of volume compensation method and is provided with a cuff, an external force applying device, and pressure and volume sensors. The pressure of liquid contained in the cuff represents the instantaneous blood pressure under the servo control or feedback control of a volume plethysmogram. The apparatus has been improved by enabling an automatic measurement and by providing a DC level adjustment and first gain control circuit. The circuit is supplied with an output from the volume sensor, and the output is subjected to DC level adjustment and amplitude control. In addition to the above, the apparatus is automatically operated under control of a micro processor.