Document No. 34 (34号文書, Sanjūyon-gō Bunsho) is the name given the notes compiled in a scrapbook by Takano Miyo prior to the events Tsumihoroboshi-hen. It explains some of the things behind the Hinamizawa Syndrome. It is called 34 because "Miyo" is written using the characters for 3 (三) and 4 (四), making the name a tribute to the person who did the research.

Before her "death", Miyo left her notes to Ryūgū Rena. These notes strongly influenced Rena's actions for the remainder of Tsumihoroboshi-hen.

Takano's scrapbooks also share a similar name with the Emergency Manual 34 used by the prime minister.

The information in the document details a story that in the distant past a UFO had crashed and sank in Onigafuchi Swamp. It brought with it a parasitic bacteria that spread amongst the villagers. The villagers went mad from the infection and became "demons" or "oni". An alien on the ship noticed this and appeared to the villagers in what is referred to in the stories as the "Descent of Oyashiro-sama", and used advanced technology to help repress the effects of the parasite temporarily. This "Oyashiro-sama" then put in place rules to help contain the problem. Since the bacteria only really thrive in the local climate, leaving caused the symptoms to worsen, and the rule was put down that nobody should leave the village. The three great houses, Kimiyoshi House, Sonozaki House, and the Furude House, worshiped and protected him for years in the Furude Shrine.

The parasite stabilized over the years and the incident was largely forgotten. As the incident was the basis for their control and power in the village, Miyo's notes indicate a suspicion that they were looking for a way to revive the parasite and maintain the balance of power. This suspicion was the basis for Rena declaring the three great houses' intent to launch a bio-terrorist attack during the hostage situation at the school. In the document, Miyo also successfully predicted Great Hinamizawa Disaster and cited a number of details that are believed to lend credence to the document as a whole.

After reading Document No. 34, Ōishi develops a slightly different theory. He believes that, instead of an alien parasite, the families were researching an ancient poisonous pathogen. This pathogen was the true nature of what was commonly referred to as the "Curse of Oyashiro-sama". The result of their research was referred to as the "Advent of Oyashiro-sama". Rather than being able to regain their dwindling influence, they ended up losing control and wiping out the village in the Great Hinamizawa Disaster.