A liquid injection device, comprising:a pair of a rotor and a stator relatively rotatable in a face-contacting relationship under a liquid-tight state;a plurality of small openings penetrating through the rotor or stator and communicating to a plurality of liquid systems; anda plurality of bridging grooves or openings provided on the contacting surface of the rotor or stator to communicate two or more of the small openings so as to form a liquid flowing path. The rotor can be rotated to a predetermined angle in a normal or reverse direction with respect to the stator so as to communicate a desired number of small openings or cut off the communication between a desired number of small openings by the relative shift of the bridging grooves with respect to the small openings.