In an ion elution unit, a drive circuit applies a voltage between electrodes to elute metal ions from the electrodes. Polarities of the electrodes are reversed cyclically with a voltage application halt period placed in-between. A current detection circuit detects the current flowing between the electrodes. A check of operation of the current detection circuit is carried out before the application of a voltage to the electrodes is started. The operation of the current detection circuit is started when a predetermined period of time passes after the application of a voltage to the electrodes is started.