Dec 30 (Reuters) - SoftBank Group has completed its $40 billion investment in OpenAI, CNBC reported on Tuesday, citing people familiar with the matter, marking one of the largest private funding rounds ever and deepening founder Masayoshi Son's bet on AI.

SoftBank has been building one of the largest private technology investment programs in the world, with a particular focus on artificial intelligence and related infrastructure such as data centers.

AI has become the central axis of global technology markets this year, driving a surge of investment by the world's largest companies and reshaping investor expectations.

The Japanese conglomerate and the ChatGPT maker did not immediately respond to Reuters requests for comment.

In March, SoftBank agreed to invest up to $40 billion into a for-profit subsidiary of OpenAI, with the funding structured as a combination of direct capital and syndicated co-investment from other backers.

The deal valued OpenAI at around $300 billion on a post-money basis, but a later secondary stock sale completed in October, valued the company at around $500 billion, according to Pitchbook data.

SoftBank Group sent over the final $22 billion to $22.5 billion investment last week after it had syndicated $10 billion and invested $8 billion in OpenAI, the report said.

OpenAI has emerged as a central pillar of that industry-wide AI spending push.

The ChatGPT maker along with Oracle and other stakeholders has planned a project dubbed "Stargate", a vast, multi-year data-center initiative aimed at supporting next-generation AI models, with backing from major investors including SoftBank.