To provide a light intensity modulation device capable of generating an intensity-modulated optical signal multiplexed in time-division manner without increasing an amplitude and a bit rate of the modulated signal.

An input data Ai is transformed (12) into two kinds of data (B1, B2) having a half bit rate and in which phase is deviated by π with each other; the two kinds of data (B1, B2) are transformed (14) into modulated signals (C1, C2); and the light (Ec, Ep) from light sources (22, 23) are modulated (18) by the two kinds of modulated signals (C1, C2). In another configuration, an input data Ai is transformed (12, 32) into (M) kinds of data (B1-BM) having 1/M bit rate and in which phase is deviated by 2π/M with each other; the (M) kinds of data (B1-BM) are transformed (15, 35) into positive phase modulated signals (C1-CM) and their opposite phase modulated signals (D1-DM); and the light (Ec, Ep) are modulated by these (M) kinds of modulated signals (B1-BM, D1-DM).