。
That building is the tallest in Japan.
Literal
That building [topic-は] Japan-1 tall building [copula-です].
日本一 '(the) number one in Japan' is a compact superlative — 日本 'Japan' + 一 'one / first' combined into a noun that can attach to almost anything ('the tallest in Japan,' 'the tastiest in Japan'). This construction extends to 世界一 '(the) best in the world' and is a highly productive superlative pattern.