A disclosure of the present specification provides a method for performing measurement by a wireless communication device. The method may comprise the steps of: receiving a synchronization signal (SS)/physical broadcast channel (PBCH) block (SSB) from a serving cell in a first period by using a first number of reception beams; performing measurement on the basis of the received SSB; and receiving downlink data from the serving cell in a second period by using a second number of reception beams, wherein the first period and the second period are included in an SS-based measurement timing configuration (SMTC) window duration, and the downlink data is considered not to be transmitted during a third period in which the number of the reception beams is changed from the first number to the second number or is changed from the second number to the first number.