The effect of Ca doping on the Li-ion conductivity and phase stability of the rock-salt-type LiBH4 phase emerging under high pressures in the range of gigapascals has been investigated. In situ electrochemical measurements under high pressure were performed using a cubic-anvil-type apparatus. Ca doping drastically enhanced the ionic conductivity of the rock-salt-type phase: the ionic conductivity of undoped and 5 mol %Ca-doped LiBH4 was 2.2 x 10(-4) and 1.4 x 10(-2) S.cm(-1) under 4.0 GPa at 220 degrees C, respectively. The activation volume of LiBH4-5 mol %Ca(BH4)(2), at 3.2 cm(3).mol(-1), was comparable to that of other fast ionic conductors, such as lithium titanate and NASICONs. Moreover, Ca-doped LiBH4 showed lithium plating-stripping behavior in a cyclic voltammogram. These results indicate that the conductivity enhancement by Ca doping can be attributed to the formation of a LiBH4-Ca(BH4)(2) solid solution; however, the solid solution decomposed into the orthorhombic LiBH4 phase and the orthorhombic Ca(BH4)2 phase after unloading the high pressure.