This invention provides a sample injector for injecting a fixed amount of a sample into a mobile phase medium and discharging the mobile phase medium with the sample. The sample injector includes a first member having a medium passage for supplying the mobile phase medium and a sample passage for supplying the sample and a second member having a discharge passage for discharging the mobile phase medium with the sample to an outside of the injector. The second member is provided in contact with the first member. The first member and the second member are configured to move to a sample charging position and a sample injection position. The sample charging position is for the sample chamber to connect with the sample passage. The sample injection position is for the sample chamber to connect with the medium passage and the discharge passage.