A nozzle for a dental handpiece is composed of a connector portion to be connected to a handpiece body, and a nozzle body, which is provided near its end with a notch of a substantially rectangular section cut out from the front face into the lateral faces, the notch forming upstream and downstream faces. The nozzle has a channel and an injection port thereof for transferring the mixture and a channel and an injection port thereof for transferring water, separate from each other. The channel for the mixture extends through the nozzle body along the front face from the connector portion to near the end, with said injection port for the mixture opened in the upstream face. The downstream face is provided with a guide for receiving the mixture injected through the injection port for the mixture and guiding the injected mixture outside the notch. With this nozzle, the mixture is properly injected onto a treatment side in subgingival treatment, while the mixture injected subgingivally is efficiently discharged spragingivally, to improve work efficiency in dental treatment.