We apply the solution for the strong CP-problem in the 4-dimensional superstring theory recently proposed by Ibáñez and Lüst to Calabi-Yau type models and study its phenomenological aspects. In Calabi-Yau type models there seem to be phenomenologically difficult problems in the axion decoupling from the neutral gauge currents and the compatibility between the proton stability and the cosmological bound on the axion. DFSZ type invisible axion mechanism which works without heavy extra colored fields may be more promising than KSVZ axion in the viewpoint of proton stability. 1 This work was partially supported by Swiss National Science Foundation and Japan Society for the Promotion of Science. Present address: Department of Physics, Kanazawa University, Kanazawa 920, Japan.