Other than ayumi's :
ARASHI - Happiness , Everything
NEWS - SUMMER TIME
BREAKERZ - SUMMER PARTY
YUI - SUMMER SONG
flumpool - Natsu Dive
Aqua Timez - Niji , Purmeria ~Hana Uta~
KAT-TUN's Taguchi Junnosuke's solo Natsu no Basho
GReeeeN - SUN SHINE !!!
Anna Tsuchiya - Brave vibration
Koda Kumi - With your smile
KinKi Kids - Natsumoyou
Kobukuro - Summer rain
Suga Shikao - Party People (I love his new song
. You can download the single already in the Asian Media Downloads section)
Yusuke - Tanpopo
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I'm quite sure there're many others out there which're great summer songs , hope this can help
