To provide a transmission signal generating device which creates an ASK (Amplitude Shift Keying) modulated signal suppressing a noise component considering that the noise component increases when digital data are converted to the ASK modulated signal transmitted on a network.

An amplifier 12 generates a relative small amplitude clock CL1, and an amplifier 14 generates a relative large amplitude clock CL2, based on a clock CL generated by a clock generating circuit 10. A switching control circuit 20 generates a control signal SW for switching circuits 16, 18 based on digital data D. The switching circuits 16, 18 select and pass either CL1 or CL2 to a LPF (Low Pass Filter) 22 based on the signal SW. A signal connecting rectangular waves of different amplitude, is input to the LPF 22, the LPF 22 smoothes the signal, generates the ASK modulated signal in which sinusoidal waves continue, and outputs it.