It has been theoretically expected that critical phenomena of helimagnetism is drastically different from those of collinear antiferromagnetism due to the existence of chiral degree of freedom. In order to verify this prediction, we carried out X-ray magnetic scattering measurements on a simplest helimagnet beta-MnO2 using a synchrotron radiation. The critical exponent beta was estimated at 0.25+/-0.05 from the temperature dependence of the scattering intensity. This value is consistent with the predicted value 0.253 for chiral system rather than with 0.346 for collinear system. (C) 2003 Elsevier Science B.V. All rights reserved.