According to one embodiment, the data transmitting device includes a storing unit, an authenticator generating unit, and a communication unit. The storing unit stores accuracy information based on an accuracy of synchronization between a first clock and a second clock. The first clock indicates a current time of the data transmitting device. The second clock indicates a current time of other device to be a destination of a data packet. The authenticator generating unit calculates a first time by adding a predetermined time to the current time of the data transmitting device, calculates a second time by adjusting the first time using the accuracy information, and generates an authenticator using the second time, the data packet, and a predetermined secret key. The communication unit transmits, to the other device, the data packet with the authenticator at a third time obtained by adding the second time to a predetermined time.