Loose forward Leitch, who captained Japan to the quarter-finals on home soil in 2019, and rake Horie were included in an initial 30-man party, to which Joseph will add three more players before the August 21st deadline.
"Considering my age, this may be my last Rugby World Cup, so I will give it my all without regrets," Horie said in a news release.
The three additional players are likely to include at least a couple of locks, with Australian-born James Moore and Jack Cornelsen the only specialist second-row forwards in the squad named on Tuesday.
The squad included two uncapped players in prop Sione Halasili and scrumhalf Kenta Fukuda, while 17 of the players will be heading to their first World Cup.
Utility back Kotaro Matsushima, who played a key role in Japan's stunning victory over South Africa in the 2015 World Cup and scored five tries on the run to the last eight four years ago, will play his third World Cup.
Pieter Labuschagne was not named in the squad after being handed a three-match ban for a dangerous tackle in Japan's 35-12 loss to Fiji in the Pacific Nations Cup in Tokyo on August 5th.
The suspension ruled the South-African born flanker out of Japan's Rugby World Cup opener against Chile in Toulouse on September 10th, although he would be free to face England in a crunch Pool D fixture a week later.
The Brave Blossoms, who also lost to Samoa but beat Tonga in the Pacific Nations Cup, also face Argentina and Samoa in their pool in France.
Japan will play a final warm-up match against Italy in Treviso on August 26th before heading to their World Cup team base in Toulouse.
Squad:
Forwards: Keita Inagaki, Craig Millar, Sione Halasili, Koo Ji-won, Shinnosuke Kakinaga, Asaeli Ai Valu, Shota Horie, Atsushi Sakate, Kosuke Horikoshi, James Moore, Jack Cornelsen, Shota Fukui, Kazuki Himeno, Michael Leitch, Ben Gunter
Backs: Naoto Saito, Yutaka Nagare, Kenta Fukuda, Lee Seung-sin, Rikiya Matsuda, Jumpei Ogura, Ryoto Nakamura, Tomoki Osada, Shogo Nakano, Dylan Riley, Siosaia Fifita, Semisi Masirewa, Jone Naikabula, Lomano Lemeki, Kotaro Matsushima.
