The purpose of this research was to analyze thestudents’ ideas in teaching practice using lesson study and open approach: a case study of the area of parallelograms. This research is qualitative, the target group is the 5th-gradeLesson Studyteam for the semester 2/2020, Ban Kaeng KhroNong Phai Community School, with 4 people(2pre-service teachers and 2 co-observer in-service teachers) Collect data by participatory observations, field notes, videos, audio, still images, and students’ worksheet, and then analyze them by protocol analysis and analytical description. Analysis of students’ideas based on analysis of the anticipated ideasin the designed lesson plansand actual ideasin the classroom from the analysis of student specimens obtained from the collection of post-session activity slips. The results showed that the projected concepts and actual concepts in the class were: 1) the concept(ideas)of finding the areaof a parallelogram; which uses counting the area of a square on a parallelogram to find the area, and 2) the concept(ideas) of calculating the area of the parallelogram. It is based on the concept(ideas) of finding the area of a rectangle from the long and wide sides, i.e. the area of the parallelogram = base x high. The students’ ideas that occur in the classroom are important to help teachers identify access to problems according to the potential of each student. Provide examples or suggestions in steps 2 and 3 of the open approach (Open Approach)to encourage students to think according to the problem approach in a learning situation that meets the goals of each lesson.

Anticipated Student’s Ideas, Lesson Study,Open Approach,Parallelograms