A liquid sample (104) introduced in a main flow passage (101) is held in a dam portion (105), and a trigger liquid (106) is filled in a trigger flow passage (102). In this state, the trigger liquid (106) is further introduced at desired timing into the trigger flow passage (102) so that the front end portion of the level of the trigger liquid (106) is advanced and the front end portion is brought to be into contact with the dam portion (105). This causes the liquid sample (104) to move to the right (downstream side) in the figure, resulting in the liquid sample (104) flowing out to the downstream side of the main flow passage (101). This means that the trigger liquid (106) provides priming to realize a liquid switch.