How to simultaneously achieve high security level, good scalability, and full decentralization is still a big issue in the design of a blockchain system. In this article, we propose Bicomp, a bilayer, scalable Nakamoto consensus protocol that is based purely on decentralized Nakamoto consensus. Bicomp works to generate two types of blocks - microblocks for concurrent in-network transaction serialization and macroblocks for chain formation. A leader is elected at the beginning of each round through Proof of Work-based computation; it puts microblocks mined by different nodes into one macroblock at the end of its tenure. Such bilayer design limits a leader's power and encourages as many nodes as possible to concurrently participate in the transaction serialization process, promoting the sharding nature of the system. Furthermore, several mechanisms are elaborately designed to reduce the transaction overlapping and further limit a leader's power, including a novel transaction diversity-based metric for deciding the branch legality. Security issues and potential attacks to Bicomp are extensively discussed. Experimental results based on 500 nodes all over the world show that Bicomp achieves high throughput performance.