  Dark photons have been considered potential candidates for dark matter. The
dark photon dark matter (DPDM) has a mass and interacts with electromagnetic
fields via kinetic mixing with a coupling constant of $\chi$. Thus, DPDMs are
converted into ordinary photons at metal surfaces. Using a millimeter-wave
receiver set in a radioshielding box, we performed experiments to detect the
conversion photons from the DPDM in the frequency range 10--18 GHz, which
corresponds to a mass range 41--74 $\mu\mathrm{eV}$. We found no conversion
photon signal in this range and set the upper limits to $\chi <
(0.5\text{--}3.9) \times 10^{-10}$ at a 95% confidence level.
