my mikrotik rb750gr3 keeps rebooting often with message, "router rebooted without proper shutdown, probably power outage" about in every 30 mins.
But actually there's no power outage.
Please help me on this.
See the second screen shot, the uptime is 3 minutes but there;s no other logs there about it.

please help me on this.

Try to replace power adapter. Get one rated similarly to original one (24V, 0.38A) or slightly higher (bigger amperage). Or any power adapter providing voltage in allowed range (8-30V) with adequate output power - rated max power consumption of hEX is 5W without attachments (whatever that means) or 10W (absolute maximum). Personally I'd go with 24V 0.5A.

Replacing the power adapter worked very well, I used one with 12v @ 1amp's . Its Working Very Well. Thank You.

It might not always be hardware related problem. I had this problem recently with 6.43.14 on RB1100AHx4, which has redundant dual power supplies and is powered from two separate UPS devices - so there is practically no possibility for real power outage, because other devices which are fed from the same power supply worked fine.

I thought at first, that this might point to some kind of hardware problem, but then two other 1100AHx4 on different locations devices did the same thing - at least once after 60+ days uptime. I did update on these routers and it will be seen, if this thing will be happening again...

@nmt1900 is right, power adapter is not always the cause of unexplained device reboots. But it is causing the problem quite often, specially with the low-end devices (such as hEX). If no other change on the system can explain reboots, it's worth to explore this path.