  We investigate a possibility of estimating mass of an isolated rapidly
rotating neutron star (NS) from a continuous gravitational wave (GW) signal
emitted by the NS. When the GW passes through the gravitational potential of
the NS, the GW takes a slightly longer time to travel to an observer than it
does in the absence of the NS. Such a time dilation effect holds also for
photons and is often referred to as the gravitational time delay (or the
Shapiro time delay). Correspondingly, the phase of the GW from the NS shifts
due to the Coulomb type gravitational potential of the NS, and the resulting
logarithmic phase shift depends on the mass, the spin frequency of the NS, and
the distance to the NS. We show that the NS mass can, in principle, be obtained
by making use of the phase shift difference between two modes of the continuous
GW such as once and twice spin frequency modes induced by a freely precessing
NS or a NS containing a pinned superfluid core. We estimate the measurement
accuracy of the NS mass using Monte Carlo simulations and find that the mass of
the NS with its ellipticity $10^{-6}$ at 1 kpc is typically measurable with an
accuracy $20%$ using Einstein Telescope.
