A sample injection device comprises a sample injection portion that is connected to a column in order to inject a sample into the column, a sample injection needle that is configured to be attachable to the sample injection portion, a sample suction means that is connectable with the sample injection needle and causes the sample injection needle to suck a predetermined amount of sample when connected therewith, a mobile phase supply means for supplying a mobile phase to the column, a first switching valve for selectively connecting the sample injection needle to either of the sample suction means or the mobile phase supply means, and a second switching valve that is provided with the sample injection portion in order to supply the sample and the mobile phase to the column via the sample injection needle when the sample injection needle is attached to the sample injection portion and in order to supply the mobile phase to the column via the first switching valve when the sample injection needle is detached from the sample injection portion.