An apparatus for measuring biological data such as blood pressure of a user has a pulse detector for simultaneously obtaining fingerprint data and a first signal from the user's body such as a pulse wave signal. The fingerprint data are used to check if the user is a registered user. After the user's biological data such as pulse waves are measured and a second signal is obtained from the user, the first and second signals are compared to check if this is the same user from whom the fingerprint data were taken in order to prevent the apparatus from being used by a wrong person. Comparison may be made between the first signal and a pattern of pulse waves extracted from earlier obtained pulse waves stored in a memory. Similar components may be used for an exercising machine to prevent it from being used by a person not preliminarily registered or a person different from the one who passed the fingerprint test on the machine.