A puncture needle assembly (10) comprising: a puncture device (13) which is provided with a needle tube (5) having a sharp needlepoint at the tip thereof, and also with a protector (7) which is supported relative to the needle tube (5) so as to be movable in the axis direction of the needle tube (5) between a first position at which the protector (7) covers at least the needlepoint of the needle tube (5) and a second position at which the protector (7) exposes the needlepoint and is located further on the base end side than the first position a cap (14) which is removably mounted to the puncture device (13) lock means (681, 722) which can, when the protector (7) is at the first position, maintain the position of the protector and a lock release means (8) which, when the protector (7) moves from the first position to the second position, enables the protector (7) to move by releasing the locked state maintained by the lock means (681, 722). The cap (14) has a prevention section (145) which, in a cap-mounted state in which the cap (14) is mounted on the puncture device (13) with the protector (7) located at the first position, prevents the lock means (681, 722) from being unlocked.
