Designing composite electrodes with optimal microstructure, composition, and choice of active material (AM) as well as solid electrolyte (SE) is critically important for the development of high-performance solid-state batteries (SSBs). To optimize AM loading, which includes loading amount, composition, and dispersion state, and to maximize AM utilization in composite SSB electrodes, we need to precisely understand how the AM loading affects electrochemical reactions taking place in the electrodes. Here, using computed tomography combined with X-ray absorption near edge structure spectroscopy (CT-XANES), we performed operando three-dimensional (3D) observations of electrochemical reactions in composite SSB electrodes with different AM loading amounts to understand the influence of the AM loading on the electrochemical reactions. In the composite electrode with higher AM loading amount, the lower reacted regions were mainly found at the inner parts of the aggregated AM regions. It was suggested that such a reaction distribution resulted from the slow intergranular ion transport between AM particles. In the composite electrode with lower AM loading amount, the electrochemical reaction progressed more homogeneously compared to the one with higher loading. This is probably because the lower AM loading mitigated the AM aggregation and decreased the number of high-resistance AM-AM interfaces that Li ions must pass through. Such a reaction distribution formation due to the slow ion transport between the AM particles can be a serious restriction in composite SSB electrodes. This is in marked contrast to conventional liquid-based lithium ion battery (LIB) electrodes, in which a majority of AM particles can directly exchange Li ions with the surrounding liquid electrolyte. Therefore, the optimal design for composite SSB electrodes can significantly differ from that for liquid-based LIBs. Our analysis technique can provide valuable information to rationally design optimal composite electrodes, and hence we expect that this technique contributes to the further development of high-performance SSBs.