I managed to enter the Japan Airlines(JL) DIAMOND PREMIER LOUNGE at Osaka Itami(ITM) airport.

Although I am not an elite membership of JL neither any other Oneworld member’s, how could I enter there?

I will give you a clue.
[toc]

This is why I could enter

Okay here is the clue :

JL offers “upgrade on that day” for domestic flights(Subject to availability).

https://www.jal.co.jp/dom/change/class.html
(In Japanese)

From Economy class to Class J, which is more comfortable seat and earn much more miles for only another JPY1,000.

Also they offer for the “First class”(Premium class), being served meal in a luxurious seat, from Economy class to First class costs another JPY8,000.

I got upgraded to the First class, it was available.

You can ask if it is available at the check-in counter, and if it is available you can pay for cash or credit card.

About lounge

You can get there by the lift just right after going through the dedicated security lane for the first class passenger.

The inside is like this :

It was less busy as it was early morning.

No windows though.

There were quite a lot of foods and drinks.

The socket did not have the universal function.
　
Some desks were there too:

It was very quiet and calm in the lounge.
　
Please note, they will not announce the boarding time as their policy is “quiet lounge service”, you need to keep checking your boarding time.
　
There are more space further.

You can relax everywhere.
​

How to connect to Wi-Fi

You can select ‘jal’ SSID.

No password is needed.

Then click(tap) “START”.

After reading the terms and conditions, ticking 「同意する」(“agree” in Japanese) and click(tap) 「次へ」(“next” in Japanese).

Fill in your email address.

Click(tap) 「同意する」(“agree” in Japanese), then it’ll be connected.

Speed

Uploading : 1.72Mbps
Downloading : 3.37Mbps

It had good speed but it suddenly disconnected one time.

I felt the speed was more than the number shows.

Conclusion

I think it is worth upgrading on the day.

You can have premium class experience as well as earn air miles for business class level with cheaper price.