If a quantitative research by means of administration of questionnaire survey in only one particular organisation, is that considered as case study method?
It could be a case study if you write it up and report it as a case study. For reference check Robert K. Yin, Case Study Research, Design and methods,2009. There may be other editions but his work is easy to read
If you have conducted your research in a single organisation and your research covers behavioural aspects of that organisation then obviously it falls under category of case report
It is a case study if your research questions are about the organization. Usually a case study is very complex aiming to describe the organization in your case as a whole with every thing in it from members to organizational culture etc. Usually you employ many methods and techniques in order to do that.
If you just employed a questionnaire to find out different things about the organisation' members - like opinions, attitudes - you don't have a case study. It is just a research referring to the members of a particular oganization.
I would refer you to this link on "Case study as a research method" by Zaidah Zainal. this is another link that you can use: http://managementhelp.org/evaluatn/casestdy.htm
Basically, a case study is an in depth study of a particular situation rather than a sweeping statistical survey. It is a method used to narrow down a very broad field of research into one easily researchable topic.
Whilst it will not answer a question completely, it will give some indications and allow further elaboration and hypothesis creation on a subject.
The case study research design is also useful for testing whether scientific theories and models actually work in the real world.