Despite the importance of emotions in the professional life of teachers, there is not much research on daily emotions of mathematics teachers beyond general research on mathematics anxiety of mathematics teachers and pre-service teachers in elementary school. Using the theory of cognitive structure of emotions, this qualitative case study research starts filling this gap by pursuing the following goals: (1) identify the daily discrete emotions in classroom of a high school mathematics teacher, (2) identify the situations that trigger those emotions, and (3) identify the appraisal structure that supports those emotions. Data was gathered through audiotaped self-reports in which the participating teacher reported his emotional experiences during 13 mathematics classes and one interview performed after the receipt of all self-reports. Our results show that the teacher participant experienced diverse emotions, such as satisfaction, disappointment, appreciation, happy-for, sorry-for, reproach, and anger. The triggering situations for the cognitive appraisals are about the achievement of the planned activities for the lessons and student learning. The support of the appraisals is the participants' belief that the "good attitude" of students--students' "collaboration," "independence," and "participation"--is a necessary condition to achieve participant's goals in class and for the students to learn. Our results are, in general, consistent with the research on emotions of teachers from elementary to high-school levels which shows that students' behavior in the classroom has an important impact on teachers' emotional experiences. We propose the hypothesis, for future research, that the emotional experiences of other mathematics teachers in classroom are triggered by the perceived mathematical behavior and the perceived mathematical learning of their students.