Science Ninja Team Gatchaman S01E24 - The Neon Giant Laughing in the Dark

The Science Ninja Team is invited to the International Science Organization Building at Ameris. When investigating the deaths of their professors, they encounter a large Mecha nearby. They also go to an amusement park run by a suspicious flute performer. 

The English Dub has the title: The Neon Giant That Smiles In The Dark.