It’s Kobinata 小日向！
Description from DLSite:
The main character of this story lives in a house called Fujimisô. One hot afternoon he pokes around his futon closet and discovers an old diary belonging to the previous tenant. It’s full of details recounting very specific events in his life. Our new tenant then notices that things come to pass in his own life as described in the journal. When he discovers he is also able to rewrite changes into the diary himself, he decides to write about a living with another man, in Fujimisô…
Sounds like the opposite of Death Note! We love it.
*Kobinata is also a contributor to the dojinshi anthology Otoko Matsuri available at the MASSIVE store.