A pressure sensor (1) for an extracorporeal circulating circuit comprising a liquid chamber (6), a pressure measuring means (7), and a liquid flow path (8) wherein the liquid chamber (6) comprises a reference surface (10) not deformed by a pressure in the extracorporeal circulating circuit, a deforming surface (20) disposed separated from the reference surface (10) and deformed at least partially by a pressure in the extracorporeal circulating circuit, a first connection surface (11) that joins the deforming surface (20) with the reference surface (10) to form a closed liquid-tight space inside them and is not deformed by a pressure in the extracorporeal circulating circuit, a liquid inflowing port (40) provided in the side surface of the first connection surface (11), and a liquid outflowing port (41) disposed away by 1/2 to less than one round from the inflowing port (40) in the flowing direction of liquid introduced along the inner periphery of the side surface of the first connection surface (11) the pressure measuring means (7) measures the deformation amount of the deforming surface (20), and is disposed outside the liquid chamber (6) and the liquid flow path (8) is liquid-tightly connected with the liquid inflowing port (40) so that liquid to be introduced into the liquid chamber (6) flows in along the inner periphery of the side surface of the first connection surface (11).
