When a packet data is transmitted, a sync pattern comprising `q` words (`q`=m+n) is added to the data. The `q` words is formed by combining a fixed pattern comprising `m` words (`m` is an integer greater than 0) with plural types of variable patterns each comprising `n` words (`n` is an integer greater than 1). In this case, a bit structure included in at least two consecutive packets should include different types of variable patterns.