43. Are there limits on the amount that third parties can spend on election campaign activities?

Source: 
政治資金規正法, 1948 (2022改正)
Political Funds Control Act, 1948 (as amended by Act No.68 of 2022)

第四条　３　この法律において「寄附」とは、金銭、物品その他の財産上の利益の供与又は交付で、党費又は会費その他債務の履行としてされるもの以外のものをいう。

Article 4. 3. In this Act, “donation” means the provision or delivery of money, goods, or other property interests, excluding those made as payment of party dues, membership fees, or other debts.

Spending by a third party is regarded as a donation, or the provision of money, goods, or services.  Therefore, the same provision for the limit on donations (as noted in 16 and 18) applies.