An on-vehicle communication device according to the present invention includes initial signal estimating unit 243b for estimating initial data based on a part of the transmission command signal from the mobile device. Thus, it also includes PN signal generating unit 243c for generating a PN signal based on the initial data estimated by the initial signal estimating unit, and partial correlating unit 243d for calculating a correlation value between the transmission command signal from the mobile device and the PN signal. Thus, it also includes a data demodulating unit 244 for demodulating the transmission command signal from the mobile device, the starting of the data demodulating unit being controlled based on the correlation value calculated by the partial correlating unit 243d.