According to one embodiment, there is provided a transmission device including a digital PLL and a power amplifier. The power amplifier is connected to the digital PLL. The digital PLL includes a digital oscillator and a controller. The controller operates the digital oscillator with a first loop bandwidth in a first period corresponding to startup of the power amplifier, operates the digital oscillator with a second loop bandwidth narrower than the first loop bandwidth in a second period being after the first period, and operates the digital oscillator with a third loop bandwidth narrower than the second loop bandwidth in a third period being after the second period.