  Pulsar timing array (PTA) experiments are expected to detect nano-Hertz
gravitational waves (GWs) emitted from individual inspiralling supermassive
binary black holes (SMBBHs). The GW signals from a small fraction of these
SMBBHs may be diffractively lensed by intervening galaxies. In this paper, we
investigate the diffractive lensing effects on the continuous GW signals from
the lensed SMBBHs and estimate the detectable number of such signals by PTAs,
such as the Chinese PTA (CPTA) and the Square Kilometer Array (SKA) PTA. We
find that the amplitude of the lensed GW signals may be only amplified by a
factor of $\sim 1.01-1.14$ ($16\%-84\%$ range) and the phase of the signals may
shift somewhat due to the lensing, significantly different from those strongly
lensed high frequency GW signals from compact binary mergers in the geometric
optics. We estimate that $\sim 0.01\%$ of all detected nano-Hertz GW signals
from individual SMBBHs by future PTA experiments are lensed by foreground
galaxies (i.e., up to $\sim 106$ for CPTA and up to $\sim 289$ for SKA-PTA).
However, the lensed nano-Hertz GW signals are difficult to be distinguished
from those without lensing by the PTA observations only. We further discuss the
possibility about the identification of the lensed nano-Hertz GW signals from
SMBBHs via the electromagnetic detection of their host galaxies or active
galactic nuclei.
