According to one embodiment, a phase estimating device includes a periodic signal obtaining unit that obtains a first periodic signal, and a wireless time synchronizing unit that synchronizes a reference time with that of a signal generating device by wirelessly communicating with the signal generating device that outputs a second periodic signal according to phase information. The phase estimating device includes a reference time storing unit that stores the reference time synchronized with that of the signal generating device by the wireless time synchronizing unit. The phase estimating device includes a phase determining unit that obtains sampled times from the reference time storing unit at timing at which the first periodic signal rises above or falls below a predetermined level, and determines phase information on a phase of the first periodic signal based on the obtained sampled time and period information on a period of the first periodic signal.