The magnetic flux density change ΔB caused by the inverse magnetostrictive effect is key for achieving high-performance vibration power generation. As the magnetization curve of the polycrystalline Fe–47.6 at. % Co–2.3 at. % V (Fe–Co–V) alloy became easier to magnetize by applying tensile stresses, the value of ΔB estimated from the magnetization curves depended significantly on magnetic fields. Hence, the vibration power generation of a U-shaped unimorph device using a polycrystalline Fe–Co–V alloy core was demonstrated under various bias magnetic fields. As a result of bias magnetic field adjustment, the open-circuit voltage induced by the vibration of the device improved to ∼7.0 V. Such superior performance is attributable to a large ΔB of 1.1 T in the Fe–Co–V alloy core. Therefore, adjustment of bias magnetic fields is essential for obtaining large ΔB in Fe–Co–V alloys, which are promising inverse magnetostrictive materials for high-performance vibration power generation.