Physical-layer security is desirable for visible light communication (VLC) in order to hide information from unauthorized receivers, without reliance on upper-layer encryption. In this paper, we propose a physical-layer secure coding scheme based on polar codes to achieve secrecy capacity of indoor VLC under Wyner's wiretap model. Compared with the beamforming approach requiring multiple-input, single-output (MISO) VLC, the proposed scheme can obtain security and reliability simultaneously for indoor single-input, single-output (SISO) VLC with binary-input discrete memoryless channels (BDMCs). Numerical results show that when the wiretap channel is degraded with respect to the main channel, the secrecy capacity can be asymptotically achieved to transmit the secret information.