There is provided a protective filter (1) for an extracorporeal circulation circuit pressure monitor, having a dialysis device side housing (2), an extracorporeal circulation circuit side housing (3), and a hydrophobic filter (4) held between both the housings, which allows passage of a gas but does not allow passage of a liquid, characterized in that a plurality of ribs (27, 38) capable of supporting the hydrophobic filter (4) are respectively provided in both housings, and a protective member (5) for preventing direct contact between the ribs (38) and the hydrophobic filter (4) is interposed between the ribs (38) provided in the extracorporeal circulation circuit side housing (3) and the hydrophobic filter (4).