  X-ray magnetic circular dichroism (XMCD) signals are usually absent in
antiferromagnets. In this letter, we report the observation of spontaneous XMCD
spectra originating from the inverse triangular spin structure, or the
polarization of the cluster magnetic octupole, in the chiral antiferromagnet
Mn$_{3}$Sn thin film. The result is consistent with the recent theoretical
predictions that the inverse triangular spin structure can give rise to finite
XMCD signals in the absence of net magnetization [J. Phys. Soc. Jpn. 89, 083703
(2020) and Phys. Rev. Lett. 126, 157402 (2021)].
