problem at arduino serial

Hello.
It can be seem like not a big problem, but to me, cause of this, it takes my few seconds when I use this serial data....

I'm using qmc5883 magnet sensor , I2c expander and mega board.
I'm sure the hardware doesn't have any problem.
But when I push serial monitor button, there are always show inverse ?mark.
Forexalmpe, say, I take 2 magnet sensor data

?35a1
88b2
105c3
-123a1
238b2
-12c2

Like this.
Now always like this but, frequently this problem is occur.
Sometime, not the inverse ?mark, other strange mark was shown.
this problem is only at the first line of serial.

The reverse question mark is often a symbol used in place of an unprintable character. Try printing the same data in binary or hex and see what the value is. Maybe some sort of STX or other framing character.

this problem is only at the first line of serial.

The usual solution is to print something else to indicate that the Arduino is ready to send good data. On the receiving end, discard anything before that.

@dongjee33, you need to post your program. At the moment I am not sure if you are talking about a problem when the Arduino receives data or when it sends data.

...R
Serial Input Basics - simple reliable ways to receive data.