  Experimental techniques to manipulate cold molecules have seen great
development in recent years. The precision measurements of cold molecules are
expected to give insights into fundamental physics. We use a rovibrationally
pure sample of ultracold KRb molecules to improve the measurement on the
stability of electron-to-proton mass ratio ($\mu = \frac{m_{\rm e}}{M_{\rm
p}}$). The measurement is based upon a large sensitivity coefficient of the
molecular spectroscopy, which utilizes a transition between nearly a degenerate
pair of vibrational levels each associated with a different electronic
potential. Observed limit on temporal variation of $\mu$ is
$\frac{1}{\mu}\frac{d\mu}{dt} = (0.30\pm1.0) \times 10^{-14}$ year$^{-1}$,
which is better by a factor of five compared with the most stringent laboratory
molecular limits to date. Further improvements should be straightforward,
because our measurement was only limited by statistical errors.
