Background：There have been many reports indicating that individuals with developmental dyscalculia show difficulties in acquiring arithmetic facts（e.g., 2 ＋ 3 ＝ 5). In contrast, few studies have examined impairments in numerical concepts, a more basic level of numerical representation. This study presents an adult case of developmental dyscalculia in which numerical magnitudes up to five can be understood from Arabic numerals, whereas those for single-digit numbers greater than five and multi-digit numbers cannot. The symptoms observed in this case may provide insights into the structure of numerical concepts and the abilities required for their acquisition.

Methods：The participant was a right-handed woman in her 20s and a fourth-year university student. Numerical and calculation abilities were assessed using subtests examining basic number manipulation, numerical concepts, reading and writing of numbers, knowledge of numbers and calculations, calculation ability, and left-right orientation and finger gnosis.

Results：The participant showed impaired understanding of numerical magnitude for single-digit numbers above approximately five and difficulty grasping the meaning of multi-digit numerals. In number decomposition and composition tasks involving zero, both errors and self-corrections were observed. She had not acquired basic arithmetic facts for single-digit addition and subtraction and had difficulty recalling multiplication tables. Although she understood procedures for multi-digit written multiplication and division, she was unsure where to write the numbers. Cardinality（understanding that the last counted number represents the total quantity）and perceptual subitizing were preserved, whereas conceptual subitizing（the rapid grasp of small quantities as structured wholes）was impaired.

Conclusions：This case, showing impaired numerical concepts above five, together with previous reports, suggests that numerical concepts differ between single- and multi-digit numbers and vary even within single-digit numbers, particularly below versus above about five. Furthermore, cardinality alone may be insufficient for acquiring numerical concepts, and conceptual subitizing may also play a role.