Provided herein is a sample injection method that enables efficient injection of a trace sample solution while reducing the measurement time. A sample solution is injected into a sample loop with air layers disposed on both sides of the sample solution, and the total amount of the sample solution, including the air layers, is injected into a detector. The start and the end of data collection are determined from the detection signal intensity changes that occur upon the air layers being injected into the detector, and the velocity of the flowing liquid is increased to reduce the measurement time. A washing solution is injected after the air layer to improve the washing efficiency and reduce the washing time.