Spontaneous baryogenesis in flat directions

We discuss a spontaneous baryogenesis mechanism in flat directions. After identifying the Nambu-Goldstone mode which derivatively couples to the associated $U$(1) current and rotates due to the A-term, we show that spontaneous baryogenesis can be naturally realized in the context of the flat direction. As applications, we discuss two scenarios of baryogenesis in detail. One is baryogenesis in a flat direction with a vanishing $B-L$ charge, especially, with neither baryon nor lepton charge, which was recently proposed by Chiba and the present authors. The other is a baryogenesis scenario compatible with a large lepton asymmetry.