According to the Singapore Government, all travellers transiting through Singapore (including Singapore Citizens and PRs) via flight or ferry need to present a valid negative COVID-19 PCR test taken within 2 days before departure (Pre-Departure COVID-19 Test) to be allowed to board their flight or ferry for travel to Singapore.
Travellers to Japan via Singapore can use the prescribed format by the Japanese Government for the test, but please note that the Singapore Government will only accept “RT-PCR” test.
 
Illustration of acceptable 2-day timeline: A traveller departing for Singapore on 3 December 2021 should take a pre-departure COVID-19 test no earlier than 1 December 2021.

【Transiting through Singapore (Singapore ICA)】
https://safetravel.ica.gov.sg/transit/overview
【To all who enter Japan （Submission of Inspection Certificate）（MHLW, Japan）】
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/0000121431_00248.html