An activity state analyzer for cycling, comprising: an acceleration sensor that detects accelerations along each of three axes that are orthogonal to each other, a gyroscope sensor that detects angular velocity along each of the three axes that are orthogonal to each other, wherein the acceleration sensor and the gyroscope sensor are attachable to a lower back of a cyclist, a sensor data acquisition unit configured to acquire sensor data as output of the acceleration sensor and the gyroscope sensor; and an activity state analysis unit configured to analyze an activity state of the cyclist during cycling with a bicycle having gears based on the sensor data acquired by the sensor data acquisition unit, the activity state analysis unit is configured to determine any one of seated, standing, inertial running, walking and stop as the activity state of the cyclist during cycling, when the activity state analysis unit determines that the activity state of the cyclist during cycling is any one of seated, standing, and walking, the activity state analysis unit calculates the number of cadence of the cyclist that is related to the information on the activity state, and the activity state analysis unit further determines information on speed and gradient during activity as the activity state of the cyclist during cycling, characterized in that the activity state analysis unit is configured to determine update or not a gear value when the activity state of the cyclist during cycling is determined as seated or standing, wherein the gear value that indicates a distance of the bicycle travelling during one rotation of a crank pedal of the bicycle varies every time the cyclist changes gears, and the activity state analysis unit does not update the gear value if i) the cadence is unstable when a difference in the cadence between last time and this time is more than a 2 rpm value, ii) the slope is descending when the gradient is a negative and lower value than a predetermined value, and iii) the road surface is rough when the variance of acceleration triaxial norm is larger than a predetermined value, wherein the variance of acceleration triaxial norm that is obtained by removing component of gravity acceleration from output of the acceleration sensor. an acceleration sensor that detects accelerations along each of three axes that are orthogonal to each other, a gyroscope sensor that detects angular velocity along each of the three axes that are orthogonal to each other, wherein the acceleration sensor and the gyroscope sensor are attachable to a lower back of a cyclist, a sensor data acquisition unit configured to acquire sensor data as output of the acceleration sensor and the gyroscope sensor; and an activity state analysis unit configured to analyze an activity state of the cyclist during cycling with a bicycle having gears based on the sensor data acquired by the sensor data acquisition unit, the activity state analysis unit is configured to determine any one of seated, standing, inertial running, walking and stop as the activity state of the cyclist during cycling, when the activity state analysis unit determines that the activity state of the cyclist during cycling is any one of seated, standing, and walking, the activity state analysis unit calculates the number of cadence of the cyclist that is related to the information on the activity state, and the activity state analysis unit further determines information on speed and gradient during activity as the activity state of the cyclist during cycling, characterized in that the activity state analysis unit is configured to determine update or not a gear value when the activity state of the cyclist during cycling is determined as seated or standing, wherein the gear value that indicates a distance of the bicycle travelling during one rotation of a crank pedal of the bicycle varies every time the cyclist changes gears, and the activity state analysis unit does not update the gear value if i) the cadence is unstable when a difference in the cadence between last time and this time is more than a 2 rpm value, ii) the slope is descending when the gradient is a negative and lower value than a predetermined value, and iii) the road surface is rough when the variance of acceleration triaxial norm is larger than a predetermined value, wherein the variance of acceleration triaxial norm that is obtained by removing component of gravity acceleration from output of the acceleration sensor. i) the cadence is unstable when a difference in the cadence between last time and this time is more than a 2 rpm value, ii) the slope is descending when the gradient is a negative and lower value than a predetermined value, and iii) the road surface is rough when the variance of acceleration triaxial norm is larger than a predetermined value, wherein the variance of acceleration triaxial norm that is obtained by removing component of gravity acceleration from output of the acceleration sensor.