[Problem] To provide: an injection needle and an injector which are capable of simplifying injection and aspiration processes using an injector and reducing process time； and a co-injection device and co-injection method which perform a co-injection process using the injector. [Solution] This injection needle (11c) includes a needle tube (211), and a ventilation part (213) that connects, outside the needle tube (211), a first position (first opening (2131)) positioned nearer the rear end (211) side than the tip (2111) of the needle tube (211) and a second position (second opening (2132)) positioned nearer the rear end (2112) side than the first position.
