charcter set of search job api result

i uploaded logs with multibyte character set(Japanese syllabary characters and Chinese characters).
encoding is utf-8.

when i use search job api to download these logs, the letters in the logs got garbled.
could you tell me how to resolve this problem?

exmaple powershell scripts as follows

$query_headers = @{"Accept"="application/json"}
$query = ''_collector=_sourceCategory=skysea2/daily | csv _raw extract affiliationmasterserver,pcno,computername,ipaddress,sessionid,loginname,displayedname,date,category,alert,term,pathurl,title,description,macaddress,accesspc,accessuser,manipulationtype,sharedname,path1,drive1,device1,vendorid1,productid1,serialno1,path2,drive2,device2,vendorid2,productid2,serialno2,text,printer,printnumber,recipient,sender,attachment,drivetype,manipulation1,manipulation2,writtencontent,type,gatewaymacaddress,gatewayipaddress,info1,detail1,info2,detail2,info3,detail3,info4,detail4,sourcecomputername,sourceipaddress,applicationname,packageid | count loginname, displayedname"
$query_data =
@"
{
"query": "$query",
"from": "2015-11-10",
"to": "2015-11-11",
"timeZone": "JST"
}
"@

$res1 = Invoke-RestMethod -Uri "https://api.au.sumologic.com/api/v1/search/jobs" -SessionVariable session_data -Headers $query_headers -Body $query_data -ContentType "application/json" -Method Post
$res2 = Invoke-RestMethod -Uri "https://api.au.sumologic.com/api/v1/search/jobs/##########/messages?offset=0&limit=100" -WebSession $session_data -ContentType "application/json" -Method Get