  We consider the sterile neutrino, that is also know as heavy neutral lepton
(HNL), interacting with the Standard Model (SM) active neutrino and photon via
a transition magnetic moment, the so-called dipole portal, which can be induced
from the more general dipole couplings which respect the full gauge symmetries
of the SM. Depending on the interactions with ${\rm SU}(2)_L$ and ${\rm
U(1)}_Y$ field strength tensors ${\cal W}_{\mu \nu}^a$ and $B^{\mu \nu}$, we
consider four typical scenarios and probe the constraints on the couplings with
photon $d_\gamma$ at LEP using the analyses to search monophoton signature and
the measurement of $Z$ decay. We find that in the considered scenarios assuming
the coupling with $Z$ boson $d_Z\neq 0$, the measurement of $Z$ decaying into
photon plus invisible particles can provide stricter constraints than the
monophoton searches at the LEP1. The complementary constraints to existing
experiments can be provided by the LEP. We also investigate the sensitivity on
the dipole portal coupling $d_\gamma$ from the monophoton searches at future
electron colliders, such as CEPC, and find that CEPC can explore the previously
unconstrained parameter space by current experiments.
