We consider iterated integrals of log ζ(s) on certain vertical and horizontal lines. Here, the function ζ(s) is the Riemann zeta-function. It is a well known open problem whether or not the values of the Riemann zetafunction on the critical line are dense in the complex plane. In this paper, we give a result for the denseness of the values of the iterated integrals on the horizontal lines. By using this result, we obtain the denseness of the values of t 0 log ζ(1/2 + it )dt under the Riemann Hypothesis. Moreover, we show that, for any m ≥ 2, the denseness of the values of an m-times iterated integral on the critical line is equivalent to the Riemann Hypothesis.