  Sudoku is a famous logic puzzle where the player has to fill a number between
1 and 9 into each empty cell of a $9 \times 9$ grid such that every number
appears exactly once in each row, each column, and each $3 \times 3$ block. In
2020, Sasaki et al. developed a physical card-based protocol of zero-knowledge
proof (ZKP) for Sudoku, which enables a prover to convince a verifier that
he/she knows a solution of the puzzle without revealing it. Their protocol uses
90 cards, but requires nine identical copies of some cards, which cannot be
found in a standard deck of playing cards (consisting of 52 different cards and
two jokers). Hence, nine identical standard decks are required to perform that
protocol, making the protocol not very practical. In this paper, we propose a
new ZKP protocol for Sudoku that can be performed using only two standard decks
of playing cards, regardless of whether the two decks are identical or
different. In general, we also develop the first ZKP protocol for a generalized
$n \times n$ Sudoku that can be performed using a deck of all different cards.
