  We investigated the thermodynamic property of the heavy fermion
superconductor UTe$_2$ in pulsed high magnetic fields. The superconducting
transition in zero field was observed at $T_{\rm c}$=1.65 K as a sharp heat
capacity jump. Magnetocaloric effect measurements in pulsed-magnetic fields
obviously detected a thermodynamic anomaly accompanied by a first-order
metamagnetic transition at $\mu$$_{0}$$H_{\rm m}$=36.0 T when the fields are
applied nearly along the hard-magnetization $b$-axis. From the results of heat
capacity measurements in magnetic fields, we found a drastic diverging
electronic heat capacity coefficient of the normal state $\gamma$$_{\rm N}$
with approaching $H_{\rm m}$. Comparing with the previous works via the
magnetic Clausius-Clapeyron relation, we unveil the thermodynamic details of
the metamagnetic transition. The enhancement of the effective mass observed as
the development of $\gamma_{\rm N}$ indicates that quantum fluctuation strongly
evolves around $H_{\rm m}$; it assists the superconductivity emerging even in
extremely high fields.
