A circularly polarized omnidirectional antenna consisting of a vertical sleeve dipole and three pairs of titled parasitic elements set around it is proposed. The antenna is useful to mobile communication because the use of circular polarization allows us to suppress the effect of multi-path reflection waves (inverse rotation) caused by building walls and surface of the ground. The antenna with an omnidirectional pattern has a simple structure without a feeding network for radiating circular polarization. To understand the radiation characteristics of the proposed antenna, an approximation theory using the induced electromotive force method is introduced. As an example, using a fixed spacing of a quarter wave-length between the vertical dipole and the parasitic elements, the possibility of generation of circular polarization is examined. Then the computational results of the axial ratio and the input impedance are compared with the results of the numerical analysis using the moment method and the experimental result. The radiation characteristics of the antenna can be understood by using the approximation theory introduced here. As a summary of the study, the contour map of the axial ratio of circular polarization is depicted using the moment method. For practical design of this antenna, a small correction factor should be multiplied to the calculated results. From the experimental results, the proposed antenna has a gain of 2 dBi and 3 dB band-width with an axial ratio of about 8%.