In connection with our previous work on semi-galois categories [1,2], this paper proves an arithmetic analogue of Christol's theorem concerning an automata-theoretic characterization of when a formal power series ξ = ξnt n ∈ Fq[[t]] over finite field Fq is algebraic over the polynomial ring Fq [t]. There are by now several variants of Christol's theorem, all of which are concerned with rings of positive characteristic. This paper provides an arithmetic (or F1-) variant of Christol's theorem in the sense that it replaces the polynomial ring Fq[t] with the ring OK of integers of a number field K and the ring Fq[[t]] of formal power series with the ring of Witt vectors. We also study some related problems.1 Some background on Eilenberg theory and its relationship to semi-galois categories will be briefly summarized in §5.2 for the neccessity of our discussion. For more comprehensive texts on Eilenberg theory and its historical aspect, the reader is referred to e.g. [19] and [17]; also, for the axiomatization of Eilenberg theory based on semi-galois categories, see §1, §5 and §7 in [2].