A highly-focused laser-induced microjet by the rapid vaporization of a small mass of liquid in an open liquid-filled tube is expected to solve problems of conventional needle-free injection devices. For practical use of this microjet, in this study, we investigate experimentally the volume of the microjet V ejected from the tube in order to clarify how the volume V is determined. We conduct two experiments. Firstly, we investigate the relationship between the volume V and the maximum volume of a laser-induced bubble Ω. In our method, the laser-induced bubble grows toward one open side of the tube and pushes the liquid because the tube is closed in the other side of the tube. We anticipate that the volume of the laser-induced bubble is strongly related to the volume of the micorjet V. It is found that the volume of the microjet V is nearly equal to the maximum bubble volume Ω although V is slightly smaller than Ω in all data. Secondly, we investigate the position where the liquid pushed by the bubble breaks up. When the laser-induced bubble shrinks, the bubble partly pulls back the liquid into the tube. The liquid then breaks up at the open end of the tube. Based on aforementioned results, we elucidate the mechanism for determining the volume of the microjet V by considering the pressure evolution inside the laser-induced bubble. Furthermore we construct a model for the volume V with taking into account the volume Vbefore that describes the air volume inside the narrow tube between the open end of a narrow tube and an air-water interface. The model shows a good agreement with the experimental data. These results indicate a possible ability of the volume control in the novel needle-free injection system.