Metamagnetic transitions in the novel spin-triplet superconductor UTe$_2$ were investigated through the newly developed simultaneous measurements of magnetization and sample temperature for the field along the orthorhombic $b$-axis and close to the [011] direction, where reentrant superconductivity (RSC) is detected below and above the first-order metamagnetic transition field $H_{rm m}$. Combining the Maxwells relation and the Clausius-Clapeyron equation, we obtained the field dependence of Sommerfeld coefficient $gamma$ through the first-order metamagnetic transition. A significant enhancement of the effective mass toward $H_{rm m}$ were detected for both field directions. On the other hand, above $H_{rm m}$, the effective mass discontinuously decreases for $H~||~b$, while it discontinuously increases for $H~||~sim [011]$, which seems to make a crucial role for the RSC.
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