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We have employed a mesh experiment for back-illuminated (BI) CCDs. BI CCDspossess the same structure to those of FI CCDs. Since X-ray photons enter fromthe back surface of the CCD, a primary charge cloud is formed far from theelectrodes. The primary charge cloud expands through diffusion process until itreaches the potential well that is just below the electrodes. Therefore, thediffusion time for the charge cloud produced is longer than that in the FI CCD,resulting a larger charge cloud shape expected.  The mesh experiment enables us to specify the X-ray point of interaction witha subpixel resolution. We then have measured a charge cloud shape produced inthe BI CCD. We found that there are two components of the charge cloud shapehaving different size: a narrow component and a broad component. The size ofthe narrow component is $2.8-5.7 \mu$m in unit of a standard deviation andstrongly depends on the attenuation length in Si of incident X-rays. Theshorter the attenuation length of X-rays is, the larger the charge cloudbecomes. This result is qualitatively consistent with a diffusion model insidethe CCD. On the other hand, the size of the broad component is roughly constantof $\simeq 13 \mu$m and does not depend on X-ray energies. Judging from thedesign value of the CCD and the fraction of each component, we conclude thatthe narrow component is originated in the depletion region whereas the broadcomponent is in the field-free region.

Application of the Mesh Experiment for the Back-Illuminated Charge-Coupled Device: I. Experiment and the Charge Cloud Shape