Conformational change of a helical polymer molecule induced by periodic modulation of the internal coordinates around constant values of a uniform helix was studied by numerical calculation. We paid attention to the ＇spatial resonance＇ found by Yamamoto et al., who carried out analytical calculation to a linear approximation (Yamamoto, M., Kasai, K. and Hikichi, K., J. Macromol. Sci. (Phys.), 1967, B1(2), 213). The spatial resonance means that when the wavelength of the modulation is equal to one turn of the helix, a finite amplitude of the modulation of the internal coordinates leads to divergence of the fluctuation in the external coordinates. We found that the molecular conformation in the spatial resonance induced by the modulation of the internal rotational angle or the bond angle is not a straight helix but is deformed into a ring-shaped helix, which we call ＇ringed-coil＇ conformation. On the other hand, the modulation of the bond length does not give rise to the spatial resonance. When both the internal rotational angle and the bond angle are modulated in a proper way the molecular conformation remains as a straight helix, but the atomic positions are displaced from those of the uniform helix. A few possible applications of the concept of the spatial resonance are proposed.