To provide a calculating apparatus for bicycles which enables a user to grasp data of a running state of a bicycle at an optional place.

The apparatus comprises a fixed unit 30 having a speed sensor 2 set in a bicycle and a first transmitter 3 connected to the speed sensor 2 for transmitting speed information as waves, and a calculating apparatus main body part 7 which includes a first receiver 4 for receiving waves from the first transmitter 3, a mount detect part 32 for detecting the mount state on the bicycle and a data-processing part 5 to which signals from the first receiver 4 and the mount detect switch 32 are input and which is detachably mounted on the bicycle. The decoder 5 of the calculating apparatus main body part 7 calculates a running state of the bicycle based on the signal from the first receiver 4 when judging from the signal of the mount detect switch 32 that the calculating apparatus main body part 7 is mounted on the bicycle.