The novel has been illustrated first by Katsumi Michihara (June and Crystal Bunko editions), then by Saichi Nagato (Tokuma Shoten edition). The hard cover book has not been illustrated; the cover pic and a single graphic inside the book were made by Katsumi Michihara.
Many thanks to Shayne, who found and send to me June Special with the very first edition of the novel, and to Harlequine and Kat for scanning and sharing illustrations from the Tokuma Shoten books.
These are only chosen pics from every edition. Beneath the icon tables you will find links to the full sets.
Tokuma Shoten edition (2009-10) – Saichi Nagato
Crystal Bunko edition (2001-07) – Katsumi Michihara
Shousetsu June edition (1986-87) – Katsumi Michihara
Other Michihara's pics