Abstract: A main fluid passageway is provided in a main valve body between first and second ports formed and an auxiliary passageway having its opposite ends closed is also formed in the valve main body so as to intersect the main passageway. A spool is slidably mounted in the auxiliary passageway and gap spaces are provided at the opposite ends of the spool. One of the gap spaces communicates with the first port, while the other gap space communicates with the second port. A locking section is provided on the spool and a cooperating latch is provided in the valve main body. Upon occurrence of an emergency, the spool is moved to shut off the main passageway and the latch is caused to engage the locking section resulting in generation of an alarm. Under such condition, even if the pressure of the fluid at the valve varies, the locked condition of the spool can be maintained.