Meaning and importance of proof in mathematics and education increases gradually. Therefore levels of doing proof, proof-related opinions and perceptions of the teachers and pre-service teachers who will train the students in future are of importance. Accordingly, this study aims to determine the proof-related opinions of pre-service elementary mathematics teachers who still study at different grade levels. In line with this purpose, a questionnaire developed under the title "Questionnaire for Constructing Mathematical Proof" was used to determine the pre-service teachers' opinions about proof. The questionnaire comprises 27 items based on 5 point Likert-type. In the study, developmental research method was conducted and the questionnaire was applied to 187 pre-service elementary mathematics teachers from different grade levels. As a result of the study, it was revealed that pre-service teachers have positive views about proof. Also, the study revealed that confidence of pre-service teachers in proving is lower than mental process, self-assessment and belief, and attitude factors.