A method and an instrument for separating and analyzing a very small quantity of constituents to be inspected contained in an analyte such as a class of catecholamines by liquid chromatography after labeling. The entire processes of analytical operations include principally three processes, a preparing process, an impurity elimination process and a separation process. When one sample is perceived, the sample is processed in these three processes. In this case, while one sample is being processed in an impurity elimination process, a sample preceeding the one sample is processed in a separation process, and a succeeding sample is processed in a preparing process. In other words, these three processes are performed essentially in parallel. With this, it is possible to remarkably reduce analysis time as compared with individual single sample analysis.