The most authoritative catalog of Japanese historical damaging earthquakes lists a M~6 earthquake of Jan. 11, 1859 near Iwatsuki, Saitama Prefecture, in the Kanto district of central Japan. The earthquake is said to have damaged Iwatsuki Castle considerably, and the seismic intensity there is evaluated as 5-upper on the Japan Meteorological Agency (JMA) scale. However, the only historical source of the damage is a Tokugawa shogunate official document of Mar. 9, 1859 describing that it lent the lord of the Iwatsuki Castle money for repairing the castle damage caused by a “previous earthquake.” Although the date of the “previous earthquake” is not written explicitly, the editor of the catalog considered the earthquake to be the Jan. 11 event felt strongly in a wide area of the Kanto district. Since there is no other record of damage caused by this earthquake despite that many documents were written in those days, we examined whether the “previous earthquake” is truly the 1859 Jan. 11 event or not. We discovered the historical document entitled “Sho-ammon-utsushi”, a collection of transcripts of official documents of the Iwatsuki clan, which has been unknown in Japanese historical seismology. It contains a transcript of the letter asking the Tokugawa shogunate for lending money of reconstruction following the 1855 earthquake disaster. The letter was written on Dec. 4, 1855, 23 days after the 1855 Nov. 11 Edo (present Tokyo) earthquake (M 7.0-7.1) that killed about 10,000 people mainly in Edo, about 30 km south of Iwatsuki. In the letter, severe damage in and around the castle town of Iwatsuki caused by the Edo earthquake is described in detail, which is new valuable information regarding the Edo earthquake. On the other hand, no record related to the 1859 earthquake is found in the “Sho-ammon-utsushi.” Therefore, we conclude that the “previous earthquake” is the 1855 Edo earthquake and that the 1859 earthquake was not a damaging one. By comparing the intensity distribution of ground shakings of the 1859 event with the seismic intensity distributions of various earthquakes in the Kanto district during 1919-2021, we infer that the 1859 earthquake was an M 4-class event in the depth range 40-80 km beneath the area around 36.1˚N and 139.8˚E. We evaluate the seismic intensity at Iwatsuki due to the 1855 Edo earthquake as 5-upper or 6-lower on the JMA scale, which has been unknown so far.