Hi, Haruo-
This page (click) says it was written by Tom Chapin (Harry Chapin's brother) (Note: John Forster was a co-writer). Tom has written some wonderful children's songs. I used to listen to him with my kids, many moons ago. This song is on Tom's Family Tree album, which is a gem.

-Joe-

P.S. I enjoyed exploring the Bold Horizon Website. I even found a recording of "Key of R" there.