Responses
Shinkai No Venus is the continuation from the first book. Read the first book (the link you've posted) than read this one. It'll make sense.
This story link is the story before this one. This story is a continuation.
@cece
This is the sequel of the "Venus Ni Seppun". There is new update for this (Shinkai ....)!
That is the prologue of this story
here is the complete version: http://www.mangago.me/read-manga/venus_ni_seppun/