The effect of low doses of ultraviolet (UV) irradiation on morphology changes of cell has been studied based on the observation of the cell length. It was shown that UV-irradiated cell has different behavior in comparison with non-irradiated cell. From the histogram of cell-length distribution, it was confirmed that cell cycle of non irradiated cell was 28 hours, and that cell cycle of irradiated cell with dose of $20;Jm^{-2}$ was delayed (39 hours), while irradiated cell with $40;Jm^{-2}$ and $60;Jm^{-2}$ did not divide and kept growing continuously. It was supposed that in case of $20;Jm^{-2}$ of irradiation dose, the cell cycle was delayed because the checkpoint worked in order to repair DNA damage induced by generation of pyrimidine dimer, reactive oxygen species and so on. It was also supposed that in case of $40;Jm^{-2}$ and $60;Jm^{-2}$ of irradiation dose, overgrowth was induced because the checkpoint was not worked well. The morphology of overgrown cell was similar to that of normally senescent cell. Therefore, it was considered that cell senescence was accelerated by UV irradiation with irradiation doses of $40;Jm^{-2}$ and $60;Jm^{-2}$.