After a first injection of a sample, amount of change between a highest intensity and each of two ion intensities before and after a voltage showing the highest intensity is calculated for each CE voltage. If the change is equal to or less than a threshold the CE voltage showing the highest intensity in the coarse control mode is selected as the optimal value, without performing a measurement in a fine control mode. If the change in the ion intensity exceeds the threshold, a narrower CE-voltage range and a smaller step size are determined from the measurement result obtained for the first injection of the sample, and after a second injection of the sample, the ion intensity is measured while the CE voltage is varied in the fine control mode.