Thin films of Bi-Sr-Ca-Cu-O system were prepared at the substrate temperature between 550°C and 600°C. The film deposited at 570°C showed X-ray diffraction pattern of Bi₂Sr₂Ca₂Cu₃O_y phase and had a zero resistance temperature T_c at 23K. After annealing at 570°C under the oxygen pressure of 1 atm, zero resistance temperature T_c increased to 45K, but under the pressure of 0.1 atm, the crystal structure was changed and nonconducting Ca-Cu-O phase appeared.