Thank you david3921 for your reply.
I found in eBay:
http://cgi.ebay.co.uk/ws/eBayISAPI.d...:X:RTQ:GB:1123

The seller told me that Suzuki usually uses 5 different sizes of plug:

M10-1.5
M12-1.25
M12-1.5
M14-1.25
M14-1.5

I'm pretty sure that my plug is M14 (because I already measured it), but I don't know if thread size is 1,25 or 1,5