We report the existence of a field-induced ferromagnetic transition in the magnetically ordered state (<69 K) of an intermetallic compound, Tb5 Si3, and this transition is distinctly first order at 1.8 K (near 60 kOe), whereas it appears to become second order near 20 K. The finding we stress is that the electrical resistivity becomes suddenly large in the high-field state after this transition, and this is observed in the entire temperature range in the magnetically ordered state. Such an enhancement of "positive" magnetoresistance (below 100 kOe) at the metamagnetic transition field is unexpected on the basis that the application of magnetic field should favor a low-resistive state due to alignment of spins. © 2009 The American Physical Society.