A relay apparatus according to an embodiment is for use in a mobile communication system. The relay apparatus comprises: a reception side RLC entity configured to receive data from a first communication apparatus; and a transmission side RLC entity configured to transmit, to a second communication apparatus, the data received by the reception side RLC entity. The transmission side RLC entity is configured to receive, from the second communication apparatus, a first acknowledgement indicating that the second communication apparatus has successfully received the transmitted data. The reception side RLC entity is configured to transmit, to the first communication apparatus, a second acknowledgement indicating that the reception side RLC entity has successfully received the data after waiting until the transmission side RLC entity receives the first acknowledgement.