The present invention provides a body fluid suction reservoir including a flexible bag, two plates for supporting the bag, and a spring put between the two plates. The body fluid suction reservoir further includes a lock having a hook and a release part and consisting of a member different from the two plates, by which lock the spring is held in a state compressed between the two plates, and the held state is releasable. The lock has flexibility and is held by one of the plates so as to be deformable from a first state in which the hook is engaged with the other plate to a second state in which the hook is not engaged, and the lock is urged to take the first state.