As long as food is crucial for life, a topic of food waste will never go out-of-date from academic discussions. Globally, one third of food that is produced is either wasted or lost each year. Food waste has shown negative consequences economically, environmentally and socially. Therefore, researchers from various disciplines have been trying to address food waste problems happening especially in households. However, urban lifestyle has influenced the way people eat and people are dining out more than ever. Existing research in a field of out-of-home waste are focusing on quantitative data and are lacking of qualitative studies, deep insights, and implications for service designers. Thus, this thesis aims to understand why customer waste food at restaurant by considering both personal and social influences, and propose design tool for service designers to understand current situations better and to help them generate design ideas. Five research activities including Cultural probe, Photo Diary, Ethnographic observation, Workshop, and lastly, Follow-up Interview were conducted. As a result, insights were translated into Customer Journey Map and Ideation board game, designed as a sensitizing tool for service designers. The thesis makes two following contributions. It suggested explanations of current user behaviors that could lead to food waste at restaurants, and it proposed a sensitizing tool for service designer to use in an early stage of ideation process.As long as food is crucial for life, a topic of food waste will never go out-of-date from academic discussions. Globally, one third of food that is produced is either wasted or lost each year. Food waste has shown negative consequences economically, environmentally and socially. Therefore, researchers from various disciplines have been trying to address food waste problems happening especially in households. However, urban lifestyle has influenced the way people eat and people are dining out more than ever. Existing research in a field of out-of-home waste are focusing on quantitative data and are lacking of qualitative studies, deep insights, and implications for service designers. Thus, this thesis aims to understand why customer waste food at restaurant by considering both personal and social influences, and propose design tool for service designers to understand current situations better and to help them generate design ideas. Five research activities including Cultural probe, Photo Diary, Ethnographic observation, Workshop, and lastly, Follow-up Interview were conducted. As a result, insights were translated into Customer Journey Map and Ideation board game, designed as a sensitizing tool for service designers. The thesis makes two following contributions. It suggested explanations of current user behaviors that could lead to food waste at restaurants, and it proposed a sensitizing tool for service designer to use in an early stage of ideation process.