This paper proposes a high-reliable magnetic design method considering DC-biased magnetization for a Loosely Coupled Inductor (LCI) used in interleaved three-phase boost converters. The interleaved multi-phase boost converters with an LCI are well-known as one of the attractive circuit topologies that can achieve high-power-density of the converters. However, when designing and implementing an LCI in the interleaved converter, DC-biased magnetization in the transformer part of LCI must be taken into account because magnetic saturation of the core May easily occur. This phenomenon is caused by unbalanced inductor average currents in each phase. Inserting air-gaps into magnetic paths of the transformer part of a LCI is considered as one of the solutions to prevent the magnetic saturation. Nonetheless, in this case, the volume and weight of LCI increase, and they become a matter of concern because the characteristics of an LCI goes closer to the characteristics of the independent inductors. Therefore, there is a trade-off between handling the DC-biased magnetization and downsizing the magnetic core. To optimize this trade-off, a novel magnetic design method for LCI is proposed by considering the maximum permissible percentage of unbalanced inductor average currents. This paper focuses on the three-phase coupled inductors used in three-phase interleaved boost converters. The accuracy and the effectiveness of the proposed magnetic design method are discussed from both theoretical and experimental points of view.