An active stylus for sending information in association with a change in an electric field to a sensor controller via capacitive coupling formed with a sensor, the active stylus comprising: a refill body forming a pen tip; an electrode provided near the refill body; a pen pressure detection section configured to detect a pen pressure level proportional to a pen pressure applied to the refill body; a power supply; and a signal processing section configured to operate in one of first and second modes based on power supplied from the power supply, during operation in the first mode, while supplying a first transmission signal, obtained by modulating a pulse train signal with the pen pressure level, to the electrode, determine whether or not it is necessary to switch to the second mode, during operation in the second mode, while supplying a second transmission signal, obtained by modulating a sine wave signal with the pen pressure level,, to the electrode, determine whether or not it is necessary to switch to the first mode, and switch to operation in the second mode when determining that it is necessary to switch to the second mode, and switch to operation in the first mode when determining that it is necessary to switch to the first mode, wherein during operation in the first mode, while sending the first transmission signal intermittently, the signal processing section is configured to perform detection operation to detect an uplink signal sent by a position detection device (3, 3A, 3B) using the electrode during a time period in which the first transmission signal is not sent, and to determine whether or not it is necessary to switch to the second mode in accordance with a detection result of the uplink signal., wherein during operation in the second mode, while sending the second transmission signal intermittently, the signal processing section is configured to perform the detection operation to detect the uplink signal sent by the position detection device (3, 3A, 3B) using the electrode during a time period in which the second transmission signal is not sent, and to determine whether or not it is necessary to switch to the first mode in accordance with a detection result of the uplink signal.. a refill body forming a pen tip; an electrode provided near the refill body; a pen pressure detection section configured to detect a pen pressure level proportional to a pen pressure applied to the refill body; a power supply; and a signal processing section configured to operate in one of first and second modes based on power supplied from the power supply, during operation in the first mode, while supplying a first transmission signal, obtained by modulating a pulse train signal with the pen pressure level, to the electrode, determine whether or not it is necessary to switch to the second mode, during operation in the second mode, while supplying a second transmission signal, obtained by modulating a sine wave signal with the pen pressure level,, to the electrode, determine whether or not it is necessary to switch to the first mode, and switch to operation in the second mode when determining that it is necessary to switch to the second mode, and switch to operation in the first mode when determining that it is necessary to switch to the first mode, wherein during operation in the first mode, while sending the first transmission signal intermittently, the signal processing section is configured to perform detection operation to detect an uplink signal sent by a position detection device (3, 3A, 3B) using the electrode during a time period in which the first transmission signal is not sent, and to determine whether or not it is necessary to switch to the second mode in accordance with a detection result of the uplink signal., wherein during operation in the second mode, while sending the second transmission signal intermittently, the signal processing section is configured to perform the detection operation to detect the uplink signal sent by the position detection device (3, 3A, 3B) using the electrode during a time period in which the second transmission signal is not sent, and to determine whether or not it is necessary to switch to the first mode in accordance with a detection result of the uplink signal..