The Italians had dominated much of the play in the first half with their forwards providing a stable base to attack and create space but it was not until scrumhalf Tito Tebaldi scored on the stroke of halftime that they were able to pull away.
Winger Edoardo Padovani and replacement Carlo Canna both crossed shortly after the break to put the game beyond doubt as the Italians earned a bonus point from their seven tries.
Namibia, who have now lost all 20 of their World Cup matches, still managed to give the crowd something to cheer with tries to scrumhalf Damian Stevens and wingers J.C. Greyling and Chad Plato.
The Italians now travel to Fukuoka to meet Canada on Thursday, while Namibia face South Africa in Toyota City on Saturday.
The Italians, however, suffered a significant injury blow when tighthead prop Tiziano Pasquali was carried off late in the first half after he collided with captain Sergio Parisse's shoulder when they made a tackle.
The Namibians provided an early shock with scrumhalf Stevens finishing off a sweeping movement after Italy hooker Luca Bigi had overthrown a lineout.
The try, however, seemed to wake up the Italians and they began to exert some scrum dominance with referee Nic Berry awarding a penalty try after the Namibian pack had disintegrated on their line.
The Italians then produced several line breaks but were really only able to convert twice, with flyhalf Tommaso Allan smashing over after a superb run by inside centre Luca Morisi.
Tebaldi then scored on the stroke of halftime when lock Federico Ruzza popped a no-look pass to the scrumhalf, who gave his side a 21-7 halftime lead.
The Italians put the game beyond doubt when Padovani and Canno both crossed to give them a 35-7 lead and seemingly poised to open the floodgates.
A penalty from Namibia flyhalf Cliven Loubser and Greyling's try from an attacking scrum staunched the flow but the Italians used their bench to finish strongly with replacements Jake Polledri and Matteo Minozzi both crossing. Namibia's Plato finished the scoring from a searing midfield break to give Italy coach Conor O'Shea some concerns about his side's defence.