Complete Thai Visa Guide for 2019

ここにもあるように、タイの制度はころころ変わるので、本国よりも信頼できる第三国のほうが分かり易いかも知れません。かなり主観的で、Forums, like ThaiVisaの参加者までも皮肉っていますが（これは言えているかも・・）、
www.locationindependent.co.uk
Thai Visas, Immigration, & Staying in Thailand
Thailand doesn’t help this situation, there are lots of different visas and constantly changing rules, and the governments websites are poor.

Expats and other nomads don’t help the situation, as what worked for them a few months ago might be different for you.

Blogs don’t help the situation, as they are rarely updated (and thus incorrect), or written from one persons perspective.

これ良いなあ、、、
WARNING: Thailand is no longer friendly to visitors who want to stay long term

で、漸く本論
Visa Exemption
Visa Exemption entry at Thai land border
Since January 2017, if you are resident of one of the 55 countries that get a free visa exemption on arrival (list above) you can only get this twice per calendar year. After this, you will be refused entry.

To repeat: You can only enter Thailand TWICE a year at a land border without having a visa in advance.
で、陸路は年二回のみ。恐らく、ここが問題。はっきりした決まりがない・・・・。ただし、滞在を延ばす目的と捉えられると問題となるが、三回程度で、30日以内に空路出国するのであれば、問題外なのと、次の半年に三回まで（空路）ならOKという見解から、今回は大丈夫と書こうとしたが、一回は陸路ですね。

>タイでの滞在は、いずれも数日の観光です。

ならば、入国カードには出国日とフライトが書かれているので、万が一詰問されても、滞在期限を延ばす意図がないことを証明できれば（帰国用航空券の写し）、入国拒否はないでしょう。最後の入国はすんなりは通れない可能性があることを覚悟しておけば。

How many Visa Exemptions can I use?

By land entry, you can have 2 per year – see above. By Air, there doesn’t seem to be consistently firm rules.

A Thai visa service that deals with visas every day say that when travelling by air “people using OUT/IN method to extend their stay” – so flying out the country, spending 1-2 days away, the coming back to Thailand – “might be questioned after SIX visa exempt entries. There is no specific period given and there is no rule on how many visa exempt will be issued for one person“. But six entries in one year seems to be enough to make immigration say “what is this person doing?“.

The Royal Thai Consulate in Cardiff states “Those who enter by air can only enter 3 times over a 6 month period (with a visa exemption)”. This is the first time I’ve seen this rule published, and the number seems low – if you’re a tourist hopping around Asia for a few months you might go through Thailand a few times.

Regardless, it is clear that any rules that do exist are designed to prevent people staying long term on something designed for tourists. So I think the general rule must be that using Visa Exemptions repeatedly after each other will likely give you some trouble.