For congestion control is an important aspect for PC5-based vehicle-to-everything (V2X) communication, a channel occupancy ratio (CR) has been introduced. CR is defined as the total number of sub-channels used by a user equipment (UE) for its transmissions divided by the total number of configured sub-channels over a measurement period of 1000 ms. The present invention provides a method and apparatus for calculating a CR over past multiple N subframes up to a subframe at which data is to be transmitted in sidelink. Accordingly, new sidelink transmission (or retransmission) can be considered for CR measurement.