iPhone purchased in Japan, can I use in India?

You may be able to if it supports the same bands as those used in India. However, the iPhone warranty is not international, and if support is needed, you may not be able to get service. Normally service is restricted to the country of purchase.

Keep in mind, iPhones are sold region specifically and certain features in one region may not be available in another. Also warranty usually only applies to your region of purchase, so you will have to go back to where you purchased the iPhone for service. You still maybe able to get service internationally, though the service centers are not obligated to honor the warranty. It all depends on if they have the parts for your iPhone, and at the service center’s discretion. 

Make sure it will work in your country.--> 5G and LTE. Find the iPhone that’s right for your country or region. - iPhone - Supported 5G and LTE networks - Apple

In the past iPhones were locked to specific carriers in Japan and they would not unlock them. Japan has changed the policy on this but check with your carrier about the ease in getting them to unlock the iPhone from their service if they do not sell them in unlocked mode (it is easy to buy an unlocked phone in many countries but I don't know about Japan).

From: How to unlock your iPhone for use with a different carrier - Apple Support

"You can find out if your iPhone is locked by going to Settings > General > About. If "No SIM restrictions" appears next to Carrier Lock, your iPhone is unlocked"

Check this article about carriers in Japan and check for unlocking. Wireless carrier support and features for iPhone in Asia-Pacific - Apple Support