Subtitle: Underground Superman (地底超人 Chitei Chōjin)
Undergroundmon wanted to invade the surface world for conquest. He used the chouju Gitagitanga to destroy industrial areas and help block out the sun. TAC was quick to respond to the situation and Undergroundmon that his species needed more space than they have underground. The next day, Undergroundmon summoned Gitagitanga to destroy more industrial areas. He trapped Dan Umezu, who proclaimed himself to be the "sixth Ultra Brother", and proceeded to join the Terrible-Monster. He grew to enormous size are downed TAC's planes. Hokuto summoned Ultraman Ace and Ace fought against the duo. The two monsters overpowered Ace and the sixth Ultra brother called the Ultra star to encourage Ultraman Ace. Ace destroyed Gitagitanga with the Ace Lifter. Enraged, Undergroundmon fired his laser but Ace set him on fire with a Ultra Beam. Groaning, Undergroundmon collapsed then exploded.
- Voice actor: Osamu Saka
- It is easy to notice the similarities between Undergroundmon and another alien Giron Man, who also used a monster to fight Ace and Zoffy. In fact, Gironman's suit was modified for Undergroundmon.
- Height: 2 ~ 52 m
- Weight: 180 kg ~ 44,000 t
- Origin: Underground
- Weakness: Any hit to Undergroundmon's chest will cause him great pain.
- Powers and Weapons
- Size Change: Undergroundmon can change his size at will.
- Claw Hands: Undergroundmon can use his claw hands to bash enemies in combat.
- Mouth Restrainers: Undergroundmon can emit energy restrainers from his mouth.
- Chest Beam: Undergroundmon can fire an energy beam from his chest.
- Eye Beams: Undergroundmon can fire red beams from his eyes.
|Ultraman Ace Kaiju|
|Verokron | Yapool | Chameleking | Vakishim | Garan | Aribunta | Giron Man | Brocken | Alien Metron Jr. | Doragory | Muruchi II | Gammas | Zaigon | Alien Antira | Unitang | Sabotendar | Baraba | Ace Killer | King Crab | Cattle God | Cowra | She-Devil | Hotarunga | Black Pigeon | Android Couple | King Kappa | Zemistlar | Aprasar | Aprasar Fairy | Space Mask | Black Satan | Giant Yapool | Mazaron Man | Yojo | Mazarius | Alien Orion | Sphinx | Alien Hipporit | Lunaticks | Undergroundmon | Gitagitanga | Red Jack | Baktari | Coakes | Bad Baalon | Kaiteigagan | Dreamgillas | Soundgillar | Machless | Snowgiran | Namahage | Alien Fire | Firemons | Alien Steal | Kaimanda | Shishigoran | Iceron | Woo II | Fubugirara | Onidevil | Gasgegon | Daidarahoshi | Hanzagiran | Verokron II | Yapool Woman | Univerlages | Aquarius | Alien Revole | Signalion | Geegon | Alien Simon | Jumbo King|