This paper presents SPA (Simple Power Analysis) attacks against public-key eryptosystems implemented on an FPGA platform. The SPA attack investigates a power waveform generated by a cryptographic module, and reveals a secret key in the module. We focus on chosen-message SPA attacks, which enhances the differences of operating waveforms between multiplication and squaring correlated to the secret key by using the input of particular messages. In particular, Yen showed a unique SPA attack against RSA cryp-tosystem, but no verification experiment using actual software or hardware was performed. In this paper, we implemented four-types of RSA processors on an FPGA platform in combination with two variants of the Montgomery multiplication algorithm and two different types of multipliers for SPA attacks experiments. Then we demonstrated effectiveness of various chosen-message attacks as well as Yen's method, and investigated the characteristics of the attacks depending on the hardware architectures.