was a Japanese convicted terrorist and senior member of Aum Shinrikyo, responsible for the deaths of a combined 19 people and for the production of sarin, VX nerve agent, PCP, and LSD. He served in Aum's "Ministry of Health". Prosecutors described him as "the second most important figure involved in the gas attacks by the Aum Shinrikyo cult" after Aum leader Shoko Asahara himself.
A native of Chiyoda, Tokyo, Tsuchiya joined Aum Shinrikyo at the age of 26 in 1991 after attending a yoga class, which he took as treatment for whiplash following a car accident. Tsuchiya earned his master's degree in physical and organic chemistry from the University of Tsukuba before fully committing to Aum Shinrikyo.
Tsuchiya was notably loyal to Asahara, describing himself as a "direct disciple of the guru", and referring to Asahara as .
Tsuchiya was charged with seven counts: the 1995 Tokyo subway sarin attack that killed 12 people; the June 1994 sarin attack in Matsumoto, Nagano, that killed seven; three VX gas attacks in 1994 and 1995 that killed one; production of PCP; and harboring Aum fugitives. During the trials, Tsuchiya was notably defiant, insulting the prosecutors and the gallery, and showing loyalty to Asahara.
In January 2004, the Tokyo District Court found Tsuchiya guilty of all charges except for harboring Aum fugitives and sentenced him to death. The presiding judge stated that although Tsuchiya was not directly involved in the attacks, he deserved to die for his "sheer evil". The Tokyo High Court upheld the sentence in August 2006, and the Supreme Court affirmed the ruling in February 2011, concluding that Tsuchiya had played a decisive role in the 1995 subway attack. In response, Tsuchiya said, "Natural result. I wanted to think about what I could do with a lifelong apology."
On 6 July 2018, Tsuchiya was executed along with Asahara and five other cult members.