There is no other mountain as tall as Mt. Fuji in Japan.

Literal

Mt. Fuji [as much as-ほど] tall mountain [topic-は] Japan [in-に] [contrast-は] other [at-に] not-exist [polite-ません].

~ほど高い山は...ない uses ほど for the comparison standard: 'a mountain as tall as Mt. Fuji doesn't exist.' Polite form with ありません.