A gas chromatograph, which may have a complex device configuration, capable of starting an analysis by replacing residual gas in a column with a carrier gas in starting up the gas chromatograph and by increasing the temperature of a column oven to an analytical initial temperature immediately after completion of the replacement.Upon command to start a gas chromatograph, a control unit calculates the average linear velocity of a carrier gas using a formula based on information such as the inner diameter and the length of a column, the column head pressure and the column outlet pressure, and the type of carrier gas, which are set as analytical conditions, and, for example, based on a detected temperature of a column oven and further calculates gas replacement necessary time from the average linear velocity and the length of the column. Based on the calculated time, a start time is determined.