Oct 21 (Reuters) - Foreign investors stepped up purchase in Japanese shares in the week to Oct. 15 as they took heart from a rally on Wall Street, ahead of the upcoming quarterly earnings results, while a drop in local COVID-19 infections added to investor optimism.

Foreigners bought stocks worth a net 336.81 billion yen ($2.95 billion) in the week to Oct. 15, compared with purchases of 13.84 billion yen in the previous week, data from Japanese exchanges showed.

Investors purchased 478.83 billion yen in cash equity markets but sold derivatives worth a net 142.02 billion yen.

Cross-border investors also purchased a net 1.45 trillion yen of Japanese bonds last week, finance ministry data showed.

For the July-to-Sept. quarter, corporate Japan is estimated to post 67.5% earnings growth from a year ago, which would top 35% growth estimated for broader Asia, Refinitiv data shows.

A weaker yen against the dollar also boosted demand for Japanese exporters, including Toyota Motor Corp. and Sony Group. The Japanese currency sank to a more than two-year low on Wednesday.

The Nikkei share average and the Topix index both surged over 3% last week, which marked their first weekly gain in four weeks.

Japanese investors purchased a net 1.2 trillion yen in overseas bonds last week. They also bought a net 64.7 billion yen in cross-border equities, marking a fifth straight week of net purchase, finance ministry data showed. ($1 = 113.9900 yen)

(Reporting by Gaurav Dogra, Patturaja Murugaboopathy and Anurag Maan; Editing by Giles Elgood)