It is important for students to develop a relational understanding of the equal sign, but students often have simultaneous operational and relational conceptions (i.e., SOR conceptions). This case study carefully explored how a student's conception of the equal sign changed during a classroom teaching experiment and analyzed the possibilities and difficulties in developing the student's conception of the equal sign. At first, the student demonstrated her understanding of only those equations in the form a ± b = c by using the operational conception of the equal sign. Given an unfamiliar equation in the form a ± b = c ± d = e, the student revealed the relational conception but still evaluated an incorrect equation as correct using the operational conception. Her SOR conception did not change easily despite considerable classroom discussion. When the teacher explained the meaning of the equal sign and connected it with a pan balance, the student understood the meaning of quantitative sameness of the equal sign and showed a stable relational conception. Later, in equations of the form a ± b = c ± d, the student's conception progressed to the comparative relational conception. Given the progression, this study focused on the noticeable shifts in the student's conceptions of the equal sign, and this article discusses both theoretical and pedagogical implications regarding students' conceptions of the equal sign.