Learn in the textbookBinary Numbers and Radix
DefinitionBinary subtraction (減算) subtracts digit by digit from the lowest one up, and in any digit where it can't subtract, it first borrows 1 from the next higher digit and then subtracts.
ExplanationA 1 in a higher digit equals 2 in the digit just below, so the digit that borrowed subtracts from its value plus 2. When borrowing across a run of 0s, the borrow travels up to the first digit holding a 1, and each 0 in between becomes 1. Inside a computer, subtraction is often done with the addition circuit by turning the subtrahend into two's complement (2の補数) and adding it.
AnalogyJust as you break a large coin into change when you are short, in binary one coin from the higher place breaks into exactly two.