Careful; cautious; wary — acting with prudence and vigilance to avoid danger or mistakes.
用心深い彼は、夜遅くに一人で出かけない。
Being cautious, he doesn't go out alone late at night.
用心深く鍵を二重にかけて出かけた。
I locked the door twice and left, being careful.
用心深さが功を奏して危険を未然に防げた。
Her caution paid off and she was able to prevent danger beforehand.