To obtain a driving method for ink injector in which a print dot of required size can be obtained with high print quality.

After injecting a first liquid droplet based on a first pulse signal A, a second liquid droplet is injected based on a second pulse signal B which catches up the first liquid droplet in the way of flight to produce an integral liquid droplet. The integral liquid droplet arrives at a paper surface to produce a print dot. The time to be elapsed after formation of an integral liquid droplet before arrival at the paper surface is varied by varying the difference of falling timing between first and second pulse signals A, B and then the diameter of print dot on the paper surface is measured. The diameter of print dot on the paper surface is larger by about 20% when the elapsed time is shorter than 100μsec than when the elapsed time is longer than 100μsec. Consequently, a print dot diameter of required size can be obtained and the print quality is enhanced.