Dentsu, Dentsu Digital, SoftBank Corp. and SB Intuitions Utilizing Generative AI to Create Compelling Japanese Copywriting

In the advertising industry, words continue to carry a lot of weight. From catchy headlines to persuasive product descriptions, effective copywriting forms the backbone of successful advertising campaigns. The creative process that goes into copywriting takes significant resources, however, and marketers can spend long periods of time brainstorming slogans, drafting variations and tailoring messages for different audiences.

Generative AI, used effectively as a tool by humans, can help address some of these challenges. Some companies in Japan have started to adopt generative AI in their advertising and marketing activities, using it for target research and analysis, creative production, effectiveness measurement, for example. However, many of the generative AI models now being used are based on large language models (LLMs) developed outside of Japan. This makes it difficult to generate advertising copy that captures the subtleties and nuances of the Japanese language.

Addressing this challenge while anticipating the needs of creators and clients, Japan’s largest advertising agency, Dentsu Inc., its group company Dentsu Digital Inc., SoftBank Corp. (TOKYO: 9434) and its subsidiary SB Intuitions Corp. launched a joint research initiative in September 2025 to develop a generative AI specialized in Japanese copywriting.

The roles of each company are as follows:

Dentsu: Evaluate generated advertising copy
Dentsu Digital: Provide training data for the Japanese copywriting-specialized AI
SoftBank Corp.: Provide AI computing infrastructure
SB Intuitions: Develop Japanese copywriting-specialized AI

Combining imaginative thinking with advanced linguistic expression

To create compelling Japanese advertising copy that faithfully expresses Japanese linguistic nuances, Dentsu and Dentsu Digital are combining their copywriting expertise and know-how in AI advertising solutions with SB Intuitions’ “Sarashina,” a Japanese-language optimized LLM built from the ground up with a deep understanding of language, culture and expression. Sarashina is being developed on SoftBank's AI computing infrastructure.

Sarashina

Using real advertising copy to enhance expressive performance and enable richer creative outputs, the project will conduct additional training on Sarashina. The AI will also learn to adjust the strength of tone, allowing it to adapt word choice to the objectives and target audience of marketing communications. In addition, the AI will be equipped with self-evaluation functions to refine its outputs. This will help it improve the accuracy of copy generation and enhance the overall quality of Japanese copywriting.

Image of the joint research

Through this joint research initiative, AI trained on copywriters' creative processes will combine imaginative thinking with advanced linguistic expression, making it possible to apply the outcomes not only to copywriting, but also to advertising tasks such as naming and narration, verbalization of ideas and concepts, and the advancement of marketing research and analysis.