A condition monitoring device for monitoring conditions related to a user. A monitor main unit includes a connection structure for detachably attachment to a first sensor unit. The first sensor unit includes a first connector mated with the connection structure of the monitor main unit and a second connector detachably attachable to a second sensor unit. Data associated with the monitored condition detected by the first sensor unit is provided to the monitor main unit via the first connector and the connection structure. Data associated with a monitored condition detected by the second sensor unit is provided to the monitor main unit via the second connector, the first sensor unit, the first connector, and the connection structure.