Magnetic tunnel junctions (MTJs) using half-metallic Heusler alloy Co 2MnSi electrodes show large tunnel magnetoresistance (TMR) ratio and large bias voltage dependence of tunnel conductance. We propose a spin transistor utilizing half-metallic characteristics of Co2MnSi. Fundamental structure is double tunnel junctions using Co2MnSi electrodes with gate electrode. The bias voltage dependence of tunnel conductance for the fabricated device has shown half-metallic characteristic of Co2MnSi electrodes. The TMR ratio has decreased with increasing gate voltage. This is the first observation of modulating the tunnel conductance by applying an external voltage in spin transistor using Co2MnSi.