The present study was designed to investigate the acquisition of Japanese “Joshis”, postpositional auxiliary words, in infants. The subjects were 102 infants age 4, 5 and 6 years. They were asked to make oral compositions, looking at the presented pictures. Twenty-one pictures were presented. Fourteen of them were concrete objects, and the others were geometric figures. The main results were as follows:(1) A group: developmental changes in the acquisition of Joshis were observed markedly between the ages of 4 and 5 years. Kaku Joshi...NI, GA, O, DE, E Kakari Joshi...WA, MO Fuku Joshi...KA Setsuzoku Joshi...TE, DE, KARA, NONI Kantou Joshi...YO (2) B group: developmental changes were observed markedly between the ages of 5 and 6 years. Fuku Joshi...MADE Setsuzoku Joshi...TO, NODE (3) C group: the acquisition of Joshis developed gradually with age. Kaku Joshi...TO, KARA, YA Kantou Joshi...NE