A keyboard device includes M driving circuits DC(1)˜DC(M), N transition circuits TC(1)˜TC(N), a control module, M column signal lines C(1)˜C(M), N row signal lines R(1)˜R(N) and M*N key units KU(1,1)˜KU(M,N). The control module performs a scanning process to sequentially scan the M column signal lines C(1)˜C(M) in M scan cycles scan(1)˜scan(M). If the key unit KU(k,x) connected with the k-th column signal line C(k) and the x-th row signal line R(x) is depressed, a scan voltage is transmitted from the k-th column signal line C(k) to the x-th row signal line R(x) through a switch sw(k,x) of the key unit KU(k,x). The transition circuit TC(x) connected with the x-th row signal line R(x) is turned on according to the scan voltage. Consequently, an output voltage Rout(x) from the transition circuit TC(x) has a first voltage level.