  We present the first measurement of the mass of a supermassive black hole
(SMBH) in the nearby double-barred spiral galaxy NGC 3504 as part of the
Measuring Black Holes Below the Milky Way ($M_{\star}$) mass galaxies
(MBHBM$_{\star}$) Project. Our analysis is based on Atacama Large
Millimeter/submillimeter Array (ALMA) Cycle-5 observations of the ${\rm
^{12}CO(2-1)}$ emission line. NGC 3504 has a circumnuclear gas disk (CND),
which has a relatively high-velocity dispersion of 30 \kms. Our dynamical
models of the CND yield a \Mbh~of $M_{\rm
BH}=1.02^{+0.18}_{-0.15}\times10^7$\Msun and a mass-to-light ratio in $H$-band
of \ml$_{\rm H}=0.66^{+1.44}_{-0.65}$ (\Msun/\Lsun). This black hole (BH) mass
is consistent with BH--galaxy scaling relations. We also detect a central
deficiency in the ${\rm ^{12}CO(2-1)}$ integrated intensity map with a diameter
of 2.7 pc at the putative position of the SMBH. However, this hole is filled by
a dense gas tracer ${\rm CS(5-4)}$ that peaks at the galaxy center found in one
of the three low-velocity-resolution continuum spectral correlators. The ${\rm
CS(5-4)}$ line has the same kinematics with the ${\rm ^{12}CO(2-1)}$ line
within the CND, suggesting that it is also an alternative transition for
measuring the central \Mbh~in NGC 3504 probably more accurately than the
current commonly used of ${\rm ^{12}CO(2-1)}$ due to its centralization.
