Is it OK for a professor to refuse access to individual test marks? I received a very low grade in one of my Master classes. This is not a failing grade but close to it. Such low scores are highly unusual in this  country (Japan), 90/100 or more is the norm and even a 80/100 is frowned upon sometimes. My score is much lower. Therefore, although I will obtain the credits for the class, there may be prejudice.

The strange thing is that this class was anything but hard. It consisted in about a dozen of lectures, one every week, done by different scientists. The evaluation consisted in a very small quiz (A5 sheet) at the end of each lecture, to be answered in 10 minutes. Usually the question was: `what did you think of this lecture, and how does it relate to your research topic ?’. I would answer it normally, in a few paragraphs.

Because my final grade is so low, and because I was very surprised, I asked the responsible professor for the details of my marks. I would like to know how well I performed in each individual test. Unfortunately, he refuses categorically to provide me with this info. Is this legal or common behavior? What should I do?
I read (skimmed) through the statues in Japanese law and the associated statutes it links.

Moreover, a legal case called the Toyama Daigaku incident clarifies that universities have large leeway in these matters. Here's the opinion of the Japanese supreme court:


  大学は、国公立であると私立であるとを問わず、学生の教育と学術の研究とを目的とする教育研究施設であって、……一般市民社会とは異なる特殊な部分社会を形成しているのであるから、このような特殊な部分社会である大学における法律上の係争のすべてが当然に裁判所の司法審査の対象になるものではない。


Translated:


  A university regardless of whether it is a national or public university or a private university is an education institution with the objective of the education of students and academic research ... Since these differ from normal society precisely in being founded to be a special type of society, the features of these institutions with respect to this special society, of course, do not fall within the purview of the court to investigate. 


While I think it's terrible,  I can find no basis for requiring the professor to supply you with the rationale for the grade you received. If anything, there's wide berth for professors to determine grades.

Your specific institution may have more precise rules but most of the institutions I looked at specified:


60% as passing
15 class sessions of 90 minutes each
grades based on some combination of classroom attendance, tests, and papers


(this is also the same where I am teaching).