Singal's begnning points distort after passing a butterworth bandpass filter

Hi,I used the matlab butter function to design a butterworth bandpass filter, the filtering result shows as below, the blue one is the original signal while the brown one is the signal after filtering.

Everything is fine except the beginning points of the signal after filtering is much smaller than the orignal signal, could u plz kindly help me on that?

[Star Strider]

How did you design and implement the filter?

Please post your code.