This paper proposes a Fuzzy Conjoint Need Assessment Method (FCNAM) procedure for model development justification. The failure to select the analysis method and neglect the data when completing the needs assessment, before developing a model, leads to incorrect implementation of the justification. There is a misunderstanding in the requirements analysis, and certain elements that should be considered in the model's design and development may be overlooked. Therefore, as an alternative method in the model development justification procedure, the fuzzy conjoint analysis method together with the triangular fuzzy number is used as a data analysis medium in the need assessment survey. As an implementation and evaluation of the FCNAM procedure, a need assessment survey involving 53 mathematics teachers was conducted to justify the need for the development of a student's mathematics problem-solving ability (SMPSA) measurement model. This procedure is proven to comply with the guideline need assessment, which is to successfully confirm gaps related to mathematics problem-solving ability, describe the perspective of teachers as users and practitioners of the proposed model and synthesize the level of development needs of the model involved. This paper contributes knowledge about the application of the triangular fuzzy number-based conjoint method in perception surveys and introduces a more effective model development justification procedure.