Effectiveness of vaccine that used recombinant R7 protein (rR7) as antigen that is derived from second-generation schizont (2GS) of Leucocytozoon caulleryi was verified under a field condition against chicken leucocytozoonosis. Chickens reared in a poultry farm where the chickens are attacked by leucocytozoonosis in every year were inoculated with oil-adjuvanted rR7 vaccine (O-rR7), and the immunized chickens were found to have production of antibodies against 2GS at a high level by one shot. Leucocytozoonosis was observed at post-injection. During the epidemic period of leucocytozoonosis, the unique clinical signs of the disease such as discharge of green feces and anemia, and also parasitemia were observed, however, compared to chickens in control group, those in O-rR7 vaccinated group had significantly slight symptoms (P<0.05). In addition to this, immunized chickens had better result of egg production than the unvaccinated chickens did, and the maximum difference of egg production rate, 22%, was observed at the peak of the disease. In conclusion, it is verified that O-rR7 vaccine has efficacy against leucocytozoonosis under field condition, and this vaccine can be put into practical use.