Abstract: In order to electrically apply fluoride by means of iontophoresis on a tooth an intermittent direct current is made to flow through the tooth. More particularly, the intermittent direct current has a waveform whose leading edge immediately rises to a peak and which decreases exponentially in time. As a result electrochemical polarization may be avoided and the ratio of the current conduction time to the repetition period may be increased so that the uptake efficiency of fluoride by the tooth may be considerably improved in a smooth manner without the need of impressing an excessive current on the tooth. Therefore adverse effects on nerve systems may be avoided, a patient under treatment feels no pain and no unpleasant stimulus, and the tooth's substance may be so improved as to be free from dental caries or decay.