Method for encoding/decoding image and device therefor

An image decoding method of decoding an image, the image decoding method comprising:

• obtaining, from a bitstream, information indicates a size of a largest coding unit and information related to a size of a reference coding unit;
• obtaining a plurality of largest coding units by splitting the image according to the information of the size of the largest coding unit;
• determining a plurality of reference coding units by splitting a width and height of the largest coding unit, among the plurality of largest coding units, according to the information related to the size of the reference coding unit;
• determining the reference coding unit, among the plurality of reference coding unis, as a first coding unit;
• determining sub coding units split from the first coding unit based on information indicating whether the first coding unit is split vertically or horizontally into the sub coding units and whether the first coding unit is split into two sub coding units or three sub coding units;
• and
• when the sub coding units do not split into smaller sub coding units, decoding the sub coding units to reconstruct the image,
• wherein, when the first coding unit is split horizontally into the three sub coding units according to the information indicating whether the first coding unit is split vertically or horizontally into the sub coding units and whether the first coding unit is split into the two sub coding units or the three sub coding units, a center sub coding unit at the center of the three sub coding units is allowed to be further split horizontally into three sub coding units, and is not allowed to be further split horizontally into two sub coding units nor allowed to be further split into four sub coding units by dividing a width and a height of the center sub coding unit.