The purpose of this study is to determine infants' preliminary linguistic ability. For this a pre-speech language test method has been developed. The test method is as follows. First, a definition of language is given. Then the development process in infancy is explained according to the definition. Subsequently, the test items are decided according to the above process. (1) Definition of language: It is difficult to define language because of its many aspects. Here, for the language definition, the aspects are restricted to the following based on the properties of natural language. Section-1 Situation of language i) Formation of communication ii) Understanding and grasping of situation circumstance Section-2 Development of symbolizing function Section-3 Representing the function of language (generally, articulation of babbling and functional change) (2) Development of language: the above three aspects develop and change from preterm infancy to late infancy. The development process is as follows. Section-1 i) The formation of communication is confirmed by action situations between baby and mother. The actions include a steady gaze in the early stage and talking baby talk later. Requests for instruction assistance and emotional communication appear with gradual differentiation of communication. Section-1 ii) Understanding and grasping of the situation circumstance is confirmed with response to action from mother to baby in the early stage. These are then confirmed with the ability to understand the action and situation of surroundings as time and space extensity, and to give appropriate appeal to the surroundings. Subsequently, understanding with language only is acquired by daily experience, including language and understanding of the surrounding.(ABSTRACT TRUNCATED AT 250 WORDS)