Kenichi Konishi was working under the pen name: 平木直 on this film since he was contracted with Studio Ghibli.

Oguro interview confirms him for the water-slide scene: http://www.style.fm/as/01_talk/konishi01.shtml

e-sakuga has his pen name credited on c1174 (also the water-slide) and later on at c1179 (1:09-1:19)