The word ‘just’ simply means ‘right’ or ‘righteous’. A right, fair and true judgement can only be made by a just judge. God is just because He only acts in accordance with His character - He always does what is right, fair and true (Deuteronomy 32:4). Because God is just, He wants His people to be just or ‘righteous’ too (Micah 6:8) and has provided a way to make this possible (see Justification).
See also
Justification