The blood pressure measuring device comprises a main device (1), and a separate device (2) capable of being used separately from the main device (1). The main device and the separate device have a wireless communication means (11 and 12), respectively. At least one device of the main device and the separate device has a power switch (10,17), and one device having the power switch is activated when the power switch is operated, and then the one device transmits a start signal (S3) to the other device by the wireless communication. The other device shifts periodically from a power-off state to a power-on state on its own when in the power-off state to check the start signal, and it is activated (S4) if it receives the start signal when it shifts to the power-on state. The one device transmits the start signal continuously over a period longer than a period during which the power switch is operated to activate the one device.
