Cryptographic circuits were analyzed regarding their side-channel information leaking behavior based on internal current source. Cryptographic circuits were implemented in an FPGA with registers arranged to demonstrate three known side-channel information leaking behaviors; (1) leakage is reduced by making Hamming distance (HD) at registers constant, (2) leakage increases with signal-to-noise ratio of side-channel traces, and (3) unbalance of routing path from registers to load circuits produces leakage. The implemented circuits were measured in terms of voltage fluctuation in the power distribution network for FPGA core circuit where the circuits were implemented. The measured voltage fluctuations were converted into internal current sources that were exploited to analyze the information leaking behavior by applying a side-channel analysis, correlation power analysis (CPA). The analysis confirmed that internal current source clearly demonstrated the side-channel information leaking behaviors. This results suggests that internal current source would allow to understand what parts of encryption circuits largely contribute to leak information and how to develop an efficient and low-cost countermeasure against side-channel attacks.