To advance our log Hodge theory, we introduce log real analytic functions and log C ∞ functions, define how to integrate them, and prove the log Poincaré lemma. We give better understandings of the degeneration of Hodge structure, including a geometric interpretation of the theory of SL(2)-orbits.Contents §0. Introduction §1. Log real analytic functions and log C ∞ functions §2. Local coordinates of log smooth morphisms in log C ∞ geometry §3. Log integration §4. Log real analytic functions and SL(2)-orbits §5. Higher direct images (plan and a special case)